[TargetLibraryInfo] Mark memrchr as unavailable on Windows

[TargetLibraryInfo] Mark memrchr as unavailable on Windows

Otherwise LLVM will optimise strrchr into memrchr on Windows resulting in linker error:
```
$ cat memrchr_test.c
int main(int argc, char **argv) {
    return (long)strrchr("KkMm", argv[argc-1][0]);
}

$ clang memrchr_test.c -O
memrchr_test.c:3:12: warning: cast to smaller integer type 'long' from 'char *' [-Wpointer-to-int-cast]
    return (long)strrchr("KkMm", argv[argc-1][0]);
           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
1 warning generated.
ld.lld: error: undefined symbol: memrchr
>>> referenced by D:/msys64/tmp/memrchr_test-e7aabd.o:(main)
clang: error: linker command failed with exit code 1 (use -v to see invocation)
```

Example taken from MSYS2 Discord and tested with windows-gnu target.

Reviewed By: aeubanks

Differential Revision: https://reviews.llvm.org/D134134