[clang] [analyzer] Fix 'bstring.c' test on Mingw (PR #192252)

[Martin Storsjö via cfe-commits]

mstorsjo wrote:

> We can't just `_WIN32` macro because it's not present on Mingw.

This comment is the wrong way around; Mingw _is_ windows and has got `_WIN32` defined, but it has GCC extensions and behaviours enabled and thus do produce those warnings. Hence checking for `__GNUC__` is more accurate.

https://github.com/llvm/llvm-project/pull/192252