crt: Provide __p__wpgmptr() function for VC20+ CRT import libraries

Function __p__wpgmptr() returns pointer to global variable _wpgmptr and is
available since VC20, it is available also in UCRT, and also in x86 version
of OS system version msvcrt.dll.

crtdll.dll and msvcrt10.dll do not provide global variable _wpgmptr and
neither __p__wpgmptr() function.

For remaining CRT import libraries (x64/arm32/arm64 msvcrt.dll) provide
mingw-w64 version of __p__wpgmptr() function defined via global variable
_wpgmptr.

Global variable _wpgmptr is available in all CRT libraries except UCRT.

Signed-off-by: Martin Storsjö <martin@martin.st>