Add a new calling convention FFI_EFI64, alias FFI_WIN64, on all X86_64
platforms. This allows libffi compiled on a 64-bit x86 platform to call
EFI functions.
Compile in ffiw64.c and win64.S on all X86_64 platforms. When compiled
for a platform other than X86_WIN64, ffiw64.c suffixes its functions
with _efi64, to avoid conflict with the platform's actual
implementations of those functions.
Declare a local variable to match the type of the struct field assigned
to it, rather than adding unsigned to the type. Fixes a -Wpointer-sign
warning.