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.
This eliminates the AM_CONDITIONAL ugliness, which eliminates
just a bit of extra boilerplate for a new target.
At the same time, properly categorize the EXTRA_DIST files
into SOURCES and HEADERS, for the generation of ctags.
