configure: Force mingw's ld to keep the reloc section

Binutils will always strip the relocation information from executable
files even if it needs it (dynamicbase/ASLR).  We can work around this
by using the pic-executable flag combined with setting the correct entry
point since apparently ld forgets what that should be.  This problem
affects both 32 and 64-bit mingw-w64.

We can combine the nxcompat/dynamicbase check because they were added to
binutils at the same time.

Signed-off-by: Alex Smith <alex.smith@warpsharp.info>