Fallout from 9787005846893c1d1c01cb8ac71abed91980b218.

While it doesn't make sense to pass --enable-x86asm option in a arm64
build, this is the only place where we don't check also for ARCH_X86
when we check for HAVE_X86ASM in code that is not x86-specific. As
a consequence we get:
```
Undefined symbols for architecture arm64:
  "_ff_tx_codelet_list_float_x86", referenced from:
      _codelet_list in tx.o
ld: symbol(s) not found for architecture arm64
clang: error: linker command failed with exit code 1 (use -v to see
invocation)
```