selftests/nolibc: Support "x86_64" for arch name

Building the kernel with ARCH=x86_64 works fine, but nolibc-test
only supports "x86", which causes errors when reusing existing build
environment.  Let's permit this environment to be used as well by making
nolibc also accept ARCH=x86_64.

Signed-off-by: Willy Tarreau <w@1wt.eu>
Tested-by: Ammar Faizi <ammarfaizi2@gnuweeb.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>