[Fuchsia] Not building llvm-mt when LIBXML2 is not enabled. (llvm#135877)

This patch prevents including the llvm-mt to `LLVM_TOOLCHAIN_TOOLS` in the
Fuchsia toolchain when LIBXML2 is not explicitly enabled.

Author: zeroomega

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -459,7 +459,6 @@ set(LLVM_TOOLCHAIN_TOOLS
   llvm-libtool-darwin
   llvm-lipo
   llvm-ml
-  llvm-mt
   llvm-nm
   llvm-objcopy
   llvm-objdump
@@ -481,6 +480,10 @@ set(LLVM_TOOLCHAIN_TOOLS
   scan-build-py
   CACHE STRING "")
 
+if (LLVM_ENABLE_LIBXML2)
+  list(APPEND LLVM_TOOLCHAIN_TOOLS llvm-mt)
+endif()
+
 set(LLVM_Toolchain_DISTRIBUTION_COMPONENTS
   bolt
   clang