libffi/x86: Always check __x86_64__ for x86 hosts

libffi/x86: Always check __x86_64__ for x86 hosts

The upstream libffi has

commit cb8474368cdef3207638d047bd6c707ad8fcb339
Author: hjl-tools <hjl.tools@gmail.com>
Date:   Wed Dec 2 12:52:12 2020 -0800

    libffi/x86: Always check __x86_64__ for x32 hosts (#601) (#602)

    Since for x86_64-*x32 and x86_64-x32-* hosts, -m32 generates ia32 codes.
    We should always check __x86_64__ for x32 hosts.

Since for gnux32 hosts, -m32 generates i386 codes, always check __x86_64__
for x86 hosts.

	PR libffi/101336
	* configure.host: Always check __x86_64__ for x86 hosts.