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.
win32.S: Add handling for position-independent code on Darwin

Newer versions of Darwin generate the necessary stub functions
automatically and just need a call instruction, but accomodating older
versions as well requires adding the stub.
Avoid referencing undefined ABIs on 64-bit Windows builds

64-bit Windows does not have FFI_STDCALL, FFI_THISCALL, or FFI_FASTCALL.
Fix a warning on 64-bit Windows

When sizeof(size_t) != sizeof(unsigned), adding a size_t to cif->bytes
produces a "possible loss of data" warning.  However, the size_t in
question refers to the size of a single parameter.  Use a cast to avoid
the warning.
Always use configure to detect whether global symbols need underscores

64-bit Windows already used this check; make it universal, and use it in
place of an ifdef on X86_WIN32, to handle non-Windows platforms that use
the underscore, such as Darwin.
Compile win32.S on 32-bit Darwin as well
Compile win32.S on FreeBSD
Merge pull request #105 from joshtriplett/win32-relocations

[3.1 blocker] win32.S needs to handle relocations/GOT
Merge pull request #110 from joshtriplett/w64

Fix 64-bit Windows support
Merge pull request #106 from joshtriplett/darwin-award

[3.1 blocker] Update OS X build system to include win32.S on 32-bit
Merge pull request #108 from joshtriplett/freebsd

[3.1 blocker] Fix FreeBSD support
Update copyright date and clean up README notes.
Upgrade version to 3.1-rc1