[compiler-rt] Fix building for arm64ec (#164590)
c208a23643231d0b19c6f795895a16dfe6797340 added the directive
`.att_syntax` when building for x86 architectures. However, when
building for arm64ec (a Windows target, for an ABI compatible with
x86_64), the defines for `__x86_64__` (and similar ones like
`__amd64__`) are still defined, so we need to check for `__arm64ec__`
here as well to skip it for such targets.
This matches similar existing ifdefs for x86_64/aarch64 in compiler-rt
builtins.
