Replacing realloc via define breaks libc++19.1.7:
```
In file included from Unified_cpp_gfx_graphite2_src0.cpp:29:
In file included from /build/firefox-135.0.1/gfx/graphite2/src/Collider.cpp:31:
In file included from /build/firefox-135.0.1/objdir/dist/stl_wrappers/functional:62:
In file included from /build/firefox-135.0.1/objdir/dist/system_wrappers/functional:3:
In file included from /nix/store/j34khyjjc2i4sxyxfk86bwcxlrz36z6g-libcxx-aarch64-unknown-linux-gnu-19.1.7-dev/include/c++/v1/functional:552:
In file included from /nix/store/j34khyjjc2i4sxyxfk86bwcxlrz36z6g-libcxx-aarch64-unknown-linux-gnu-19.1.7-dev/include/c++/v1/__functional/boyer_moore_searcher.h:27:
In file included from /build/firefox-135.0.1/objdir/dist/stl_wrappers/vector:62:
In file included from /build/firefox-135.0.1/objdir/dist/system_wrappers/vector:3:
In file included from /nix/store/j34khyjjc2i4sxyxfk86bwcxlrz36z6g-libcxx-aarch64-unknown-linux-gnu-19.1.7-dev/include/c++/v1/vector:3023:
In file included from /build/firefox-135.0.1/objdir/dist/system_wrappers/locale:3:
/nix/store/j34khyjjc2i4sxyxfk86bwcxlrz36z6g-libcxx-aarch64-unknown-linux-gnu-19.1.7-dev/include/c++/v1/locale:2458:26: error: no member named 'moz_xrealloc' in namespace 'std'
2458 | _Tp* __t = (_Tp*)std::realloc(__owns ? __b.get() : 0, __new_cap);
| ^~~~~
/build/firefox-135.0.1/objdir/dist/include/mozilla/mozalloc.h:82:16: note: 'moz_xrealloc' declared here
82 | MFBT_API void* moz_xrealloc(void* ptr, size_t size) MOZ_INFALLIBLE_ALLOCATOR;
```
We no longer support GCC 5.4, so we shouldn't need this.
Differential Revision: https://phabricator.services.mozilla.com/D239955
277c4c6dfb