Make unaligned loads work with sanitization.

Make unaligned loads work with sanitization.

Address sanitizer may miss memory bugs that happen during unaligned
loads. To prevent that, compilers may provide additional functions to
perform unaligned loads/stores, which allow sanitizer instrumentation
to check these accesses too. If the necessary header is available, and
AddressSanitizer is enabled, use them.

* configure.ac: Add check for <sanitizer/common_interface_defs.h>.
* src/lisp.h (UNALIGNED_LOAD_SIZE) [ADDRESS_SANITIZER]: New macro. If
the common sanitizer interface is available and address sanitization
is enabled, 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.