x86: Rewrite closures

Move everything into sysv.S, removing win32.S.  Handle all abis
with a single ffi_closure_inner function.  Move complexity of the
raw THISCALL trampoline into assembly instead of the trampoline
itself.  Only push the context for the REGISTER abi.
testsuite: Fix return_complex2 vs excessive precision

Use the previously computed rc2 to validate, rather than
recomputing a floating point result with excess precision.
x86: Add support for Complex
x86: Add support for Go closures
x86: Use win32 name mangling for fastcall functions