[compiler-rt] Fix building for arm64ec (#164590)

c208a23 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.

Author: mstorsjo

compiler-rt/lib/builtins/assembly.h

@@ -337,7 +337,8 @@
 #endif
 #endif
 
-#if defined(__ASSEMBLER__) && (defined(__i386__) || defined(__amd64__))
+#if defined(__ASSEMBLER__) && (defined(__i386__) || defined(__amd64__)) &&     \
+    !defined(__arm64ec__)
 .att_syntax
 #endif