Clean up __executable_start, monstartup when --enable-profiling.

Clean up __executable_start, monstartup when --enable-profiling.

The following changes affect the code only when profiling.
* dispnew.c (__executable_start): Rename from safe_bcopy.
Define only on platforms that need it.
* emacs.c: Include <sys/gmon.h> when profiling.
(_mcleanup): Remove decl, since <sys/gmon.h> does it now.
(__executable_start): Remove decl, since lisp.h does it now.
(safe_bcopy): Remove decl; no longer has that name.
(main): Coalesce #if into single bit of code, for simplicity.
Cast pointers to uintptr_t, since standard libraries want integers
and not pointers.
* lisp.h (__executable_start): New decl.