[MinGW] Mark a number of library functions unavailable for mingw targets

These functions were marked unavailable for MSVC targets before,
within an "T.isOSWindows() && !T.isOSCygMing()" block, but these ones
are unavailable on MinGW targets too.

This avoids generating calls to stpcpy for MinGW targets, which has
been happening since 6dbf0cfc (in
some cases).

This fixes https://github.com/mstorsjo/llvm-mingw/issues/201.

Differential Revision: https://reviews.llvm.org/D102946