Use the GCC inline assembly when building for MinGW

MinGW has a longstanding problem of providing the MSVC intrinsics that
every Windows developer expects to be there. Other projects have run
into those problems.

So instead just use the GCC inline assembly.

Change-Id: I5651f97f9a4dfbf98ebbf063f91f221eab80b224
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>