selftests/sgx: Skip non X86_64 platform

selftests/sgx: Skip non X86_64 platform

When building whole selftests on arm64, rsync gives an erorr about sgx:

rsync: [sender] link_stat "/root/linux-next/tools/testing/selftests/sgx/test_encl.elf" failed: No such file or directory (2)
rsync error: some files/attrs were not transferred (see previous errors) (code 23) at main.c(1327) [sender=3.2.5]

The root casue is sgx only used on X86_64, and shall be skipped on other
platforms.

Fix this by moving TEST_CUSTOM_PROGS and TEST_FILES inside the if check,
then the build result will be "Skipping non-existent dir: sgx".

Fixes: 2adcba79e69d ("selftests/x86: Add a selftest for SGX")
Signed-off-by: Zhao Mengmeng <zhaomengmeng@kylinos.cn>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Reviewed-by: Jarkko Sakkinen <jarkko@kernel.org>
Link: https://lore.kernel.org/all/20231206025605.3965302-1-zhaomzhao%40126.com