[libcxx] Add cast to avoid pointer casting warning on Windows (#92738)

This avoids the following build time warning, when building with the
latest nightly Clang:

warning: cast from 'FARPROC' (aka 'int (*)() __attribute__((stdcall))')
to
'GetSystemTimeAsFileTimePtr' (aka 'void (*)(_FILETIME *)
__attribute__((stdcall))')
converts to incompatible function type [-Wcast-function-type-mismatch]

This warning seems to have appeared since Clang commit
999d4f84, which restructured.

The GetProcAddress function returns a `FARPROC` type, which is `int
(WINAPI *)()`. Directly casting this to another function pointer type
triggers this warning, but casting to a `void*` inbetween avoids this
issue. (On Unix-like platforms, dlsym returns a `void*`, which doesn't
exhibit this casting problem.)