Fix arch detection ifdefs in glib/valgrind.h
Don't assume that __MINGW32__
implies x86; Windows runs on ARM/ARM64
as well, and there are mingw toolchains that target those architectures.
This mirrors how the MSVC part of the same expressions are written,
as (defined(_WIN32) && defined(_M_IX86))
and
(defined(_WIN64) && defined(_M_X64))
- not relying on _WIN32
/_WIN64
or __MINGW32__
/__MINGW64__
alone to indicate architecture.