On X86 windows, asflags weren't actually ever used for anything,
since assembling used x86asflags instead, and that flags list
had -DPIC already.

This fixes building shared libraries with clang for arm/aarch64
windows. (This wasn't an issue when in msvc mode before, since
we don't try to enable pic at all there, since the msvc armasm
assembler fails on our pic constructs there.)
Signed-off-by: Martin Storsjö <martin@martin.st>