Add support for additional memory checks using AddressSanitizer.

Add support for additional memory checks using AddressSanitizer.

When Emacs is compiled with AddressSanitizer support, enable
poisoning/unpoisoning freed/unused Lisp objects and other internal
memory management structures. If enabled, this will mark freed bytes
that have been put on free lists for future use and initially
allocated memory blocks/chunks as "poisoned", triggering an ASan error
if they are accessed improperly. Structures are unpoisoned when they
have been taken off their respective free lists. Additionally, add
optional macros for performing unaligned loads, which when enabled by
defining USE_SANITIZER_UNALIGNED_LOAD will use ASan provided functions
for loading from unaligned addresses, which may help catch bugs that
AddressSanitizer might otherwise miss.

* configure.ac: Check for the existence of address and common
sanitizer API headers.

* src/lisp.h (UNALIGNED_LOAD_SIZE): New macro. If enabled, and the
necessary sanitizer API is available, define it to
__sanitizer_unaligned_load(64|32) depending on the word size of the
architecture.

* src/fns.c [HAVE_FAST_UNALIGNED_ACCESS] (Fstring_lessp): Use
'UNALIGNED_LOAD_SIZE' to perform unaligned loads from the two strings.

* src/alloc.c (ASAN_POISON_ABLOCK, ASAN_UNPOISON_ABLOCK)
(ASAN_POISON_INTERVAL_BLOCK, ASAN_UNPOISON_INTERVAL_BLOCK)
(ASAN_POISON_INTERVAL, ASAN_UNPOISON_INTERVAL)
(ASAN_PREPARE_DEAD_SDATA, ASAN_PREPARE_LIVE_SDATA)
(ASAN_POISON_SBLOCK_DATA, ASAN_POISON_STRING_BLOCK)
(ASAN_UNPOISON_STRING_BLOCK, ASAN_POISON_STRING)
(ASAN_UNPOISON_STRING, ASAN_POISON_FLOAT_BLOCK)
(ASAN_UNPOISON_FLOAT_BLOCK, ASAN_POISON_FLOAT)
(ASAN_UNPOISON_FLOAT, ASAN_POISON_CONS_BLOCK)
(ASAN_POISON_CONS, ASAN_UNPOISON_CONS)
(ASAN_POISON_VECTOR_CONTENTS, ASAN_UNPOISON_VECTOR_CONTENTS)
(ASAN_UNPOISON_VECTOR_BLOCK, ASAN_POISON_SYMBOL_BLOCK)
(ASAN_UNPOISON_SYMBOL_BLOCK, ASAN_POISON_SYMBOL)
(ASAN_UNPOISON_SYMBOL) [ADDRESS_SANITIZER]: New macros. When
address sanitization is enabled, define them to poison/unpoison
objects.

(lisp_align_malloc): Poison newly allocated blocks on `free_ablock',
unpoison ablocks taken from it respectively.
(lisp_align_free): Poison individual ablocks when they are put on the
free list, unpoison them when an entire `ablocks' chunk is being
freed.

(make_interval): Poison interval blocks on initial allocation,
unpoison individual intervals on allocation and removal from
`interval_free_list'.
(sweep_intervals): Unpoison interval blocks before sweeping, poison
dead/unmarked intervals.

(allocate_string): Poison string blocks on initial allocation,
unpoison Lisp_Strings on removal from the free list.
(allocate_string_data): Poison `sblock' data on initial allocation,
unpoison individual `sdata' contents on allocation or removal from the
free list. Call `ASAN_PREPARE_LIVE_SDATA' on the new `sdata' struct.
(sweep_strings): Unpoison string blocks before sweeping them,
poisoning dead strings and their sdata afterwards.
(compact_small_strings): Call `ASAN_PREPARE_LIVE_DATA' on the `sdata'
to where compacted strings to moved to.
(pin_string): Call `ASAN_PREPARE_DEAD_SDATA' on `old_sdata'.

(make_float): Poison float blocks on allocation, unpoisoning
individual Lisp_Floats on allocation or removal from
`float_free_list'.
(sweep_floats): Unpoison float blocks before sweeping, poison
dead/unmarked floats.

(free_cons): Poison `ptr'.
(Fcons): Poison cons blocks on allocation, unpoisoning individual
Lisp_Cons on allocation or removal from `cons_free_list'.
(sweep_conses): Poison dead/unmarked conses.

(setup_free_list): Poison vectors put on `vector_free_lists'.
(allocate_vector_from_block): Unpoison vectors taken from the free
list, poison excess vector bytes when vectors allocated from the free
list are larger than requested.
(sweep_vectors): Unpoison vector blocks before sweeping them.

(Fmake_symbol): Poison symbol blocks on initial allocation,
unpoisoning individual Lisp_Symbols on allocation or removal from
`symbol_free_list'.
(sweep_symbols): Unpoison symbol blocks before sweeping, poisoning
dead/unmarked symbols.

(live_string_holding, live_cons_holding, live_symbol_holding)
(live_float_holding): When compiling with address sanitization and GC
poisoning enabled, return NULL if the passed address is poisoned, or
if the Lisp object it resides in is poisoned, avoiding a
use-after-poison trigger if these functions are called on a pointer
that might be referring to a now dead/swept object.

* etc/DEBUG: Add information about enabling ASan memory poisoning.