crt: Simplify C99 _Exit symbol

C99 _Exit symbol is available only in the UCRT DLL libraries.
It is not available in any crtdll/msvcr* DLL library.

Currently the _Exit symbol is provided by the libmingwex.a static library
and also by the UCRT import library. At the same time, mingw-w64 header
files provides inline function _Exit function.

Simplify the whole implementation. Remove inline function _Exit and also
remove mingwex _Exit implementation. For non-UCRT builds define just
_Exit symbol as alias to _exit symbol.

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