arch/posix: Add MemorySanitizer support

Wire this up the same way ASAN works.  Right now it's support only by
recent clang versions (not gcc), and only in 64 bit mode.  But it's
capable of detecting uninitialized data reads, which ASAN is not.

This support is wired into the sys_heap (and thus k_heap/k_malloc)
layers, allowing detection of heap misuse like use-after-free.  Note
that there is one false negative lurking: due to complexity, in the
case where a sys_heap_realloc() call is able to shrink memory in
place, the now-unused suffix is not marked uninitialized immediately,
making it impossible to detect use-after-free of those particular
bytes.  But the system will recover cleanly the next time the memory
gets allocated.

Also no attempt was made to integrate this handling into the newlib or
picolibc allocators, though that should hopefully be possible via
similar means.

Signed-off-by: Andy Ross <andyross@google.com>