libffi: Fix up x86_64 classify_argument

As the following testcase shows, libffi didn't handle properly
classify_arguments of structures at byte offsets not divisible by
UNITS_PER_WORD.  The following patch adjusts it to match what
config/i386/ classify_argument does for that and also ports the
PR38781 fix there (the second chunk).

This has been committed to upstream libffi already:
https://github.com/libffi/libffi/commit/5651bea284ad0822eafe768e3443c2f4d7da2c8f

2021-06-16  Jakub Jelinek  <jakub@redhat.com>

	* src/x86/ffi64.c (classify_argument): For FFI_TYPE_STRUCT set words
	to number of words needed for type->size + byte_offset bytes rather
	than just type->size bytes.  Compute pos before the loop and check
	total size of the structure.
	* testsuite/libffi.call/nested_struct12.c: New test.