[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 6dbf0cfcf789365493f70ae69df8a7a59be41c75 (in
Differential Revision: https://reviews.llvm.org/D102946