crt: Deduplicate mingw-w64 stat code into __MINGW_FIXED_STAT macro

Create new function-like macro __MINGW_FIXED_STAT with same pattern as
existing __MINGW_FIXED_FSTAT macro, and extract repeated stat and wstat
code into that new __MINGW_FIXED_STAT macro.

Use gcc compound statement extension to create function-like macro and
__builtin_choose_expr to choose correct narrow or wide function based on
the passed type (char* or wchar_t*). With this patterns, the whole stat and
wstat code is deduplicated to macro functions.

This allows to extend the __MINGW_FIXED_STAT code by followup changes
without duplicating the functionality again.

Signed-off-by: LIU Hao <lh_mouse@126.com>