Fixes https://github.com/dlang/dmd/issues/21271
Redefine the compatibility macro in terms of C99's `_Pragma()`
instead of ignoring it. Clang and GCC will replace `_Pragma()`
with `#pragma` directives in the preprocessed output, while
cl.exe will actually convert it back to `__pragma()`.
This is still a better situation than before as ImportC partially
supports `__pragma()`.
