[libc][LIBC_ADD_NULL_CHECKS] replace volatile deref with __builtin_trap (#123401)

Also, update the unit tests that were checking for SIGSEGV to not check for a
specific signal.

To further improve this check, it may be worth:
- renaming the configuration option/macro/docs to be clearer about intent.
- swap __builtin_trap for __builtin_unreachable, removing the preprocessor
  variants of LIBC_CRASH_ON_NULLPTR, then unconditionally using
  `-fsanitize=unreachable -fsanitize-trap=unreachable` in cmake flags when
  LIBC_ADD_NULL_CHECKS is enabled.
- building with `-fno-delete-null-pointer-checks` when LIBC_ADD_NULL_CHECKS (or
  when some larger yet to be added hardening config) is enabled.

Link: #111546

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