uml: exclude some /arch/x86 optimizations

The x86_64 UML target wants to include SSSE3 optimized
crypto code which lives under /arch/x86/crypto.

However, these are not built and this causes an error.
| ERROR: module '[...]/arch/x86/crypto/sha512-ssse3.ko' is missing.
| make[3]: *** [modules/crypto.mk:990: [...]/kmod-crypto-sha512_5.15.112-1_x86_64.ipk] Error 1

Signed-off-by: Christian Lamparter <chunkeey@gmail.com>