[compiler-rt] Move __sanitizer_mallinfo to separate header

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.

Differential Revision: https://reviews.llvm.org/D147092