win32.S: Make calls to ffi_closure_SYSV_inner position-independent

Now that non-Windows platforms include win32.S, it needs to support
building as position-independent code.  This fixes one source of build
failures on target platforms that do not allow text relocations.
win32.S: Make the jump tables position-independent

Now that non-Windows platforms include win32.S, it needs to support
building as position-independent code.  This fixes build failures on
target platforms that do not allow text relocations.
win32.S: Use shifting for multiplication rather than repeated addition

The jump table code added a register to itself twice to multiply by 4;
shift the register left by 2 instead.
Merge pull request #105 from joshtriplett/win32-relocations

[3.1 blocker] win32.S needs to handle relocations/GOT