[Build] Enable zstd ASM Huffman decoder on non-MSVC x86_64

zstd ships a hand-written huf_decompress_amd64.S that was being
disabled on all non-MSVC builds. New behavior matches zstd's own
CMakeLists which enables it on GCC/Clang x86_64 and disables it
on MSVC and non-x86 architectures.

third_party/CMakeLists.txt

@@ -241,7 +241,10 @@ target_compile_definitions(zstd PRIVATE
   XXH_NAMESPACE=ZSTD_
   ZSTD_LEGACY_SUPPORT=0
 )
-if(NOT MSVC)
+if(XE_TARGET_X86_64 AND NOT MSVC)
+  enable_language(ASM)
+  target_sources(zstd PRIVATE zstd/lib/decompress/huf_decompress_amd64.S)
+else()
   target_compile_definitions(zstd PRIVATE ZSTD_DISABLE_ASM)
 endif()