configure: Restrict --target-efi to x86_64

Setting the --target-efi option for any architecture but x86_64 results in
an error while trying to compile the tests:

$ ./configure --arch=arm64 --cross-prefix=aarch64-linux-gnu- --target-efi
$ make clean && make
Makefile:46: *** Cannot build aarch64 tests as EFI apps.  Stop.

Which might come as a surprise for users, as the help message for the
configure script makes no mention of an architecture being incompatible
with the option.

Document that --target-efi applies only to the x86_64 architecture and
check for illegal usage in the configure script, instead of failing later,
at compile time.

Signed-off-by: Alexandru Elisei <alexandru.elisei@arm.com>
Signed-off-by: Andrew Jones <drjones@redhat.com>