mallinfo is platform-specific and not specified by either posix or the C standard, but the hwasan interface unconditionally exposes __sanitizer_mallinfo which returns a struct __sanitizer_struct_mallinfo which is defined in sanitizer_platform_limits_posix.h, so this should also be available for fuchsia to provide __sanitizer_mallinfo. Fuchsia doesn't need the rest of what's in sanitizer_platform_limits_posix.h so we can just move it to its own header.
Exposing this and not forcing it to hide behind SANITIZER_INTERCEPT_MALLOPT_AND_MALLINFO fixes the test failures found after landing D145718.
It's usually a good idea to put blank lines around the # directive lines.