rtlib: Work around MinGW gcc bug 52991 on 64bit too

MinGW-w64's 32bit/64bit compilers suffer the same problem like the 32bit
MinGW.org one, and the same work-around ensures that structures like
__FB_DATADESC are packed instead of remaining unpacked.

Note: -gen gcc already does this which is good; there it never was
restricted to 32bit only.