[Fuchsia] Disable new hdrgen in Fuchsia toolchain build

We are seeing a PyYaml issue after the new libc_hdrgen was enabled
by default. This patch disables it in Fuchsia toolchain build so
we restore the builder while we are investigating.

Author: zeroomega

clang/cmake/caches/Fuchsia-stage2.cmake

@@ -334,6 +334,7 @@ foreach(target armv6m-unknown-eabi;armv7m-unknown-eabi;armv8m.main-unknown-eabi)
   endforeach()
   set(RUNTIMES_${target}_LLVM_LIBC_FULL_BUILD ON CACHE BOOL "")
   set(RUNTIMES_${target}_LIBC_ENABLE_USE_BY_CLANG ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBC_USE_NEW_HEADER_GEN OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_CXX_ABI none CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")
@@ -385,6 +386,7 @@ foreach(target riscv32-unknown-elf)
   endforeach()
   set(RUNTIMES_${target}_LLVM_LIBC_FULL_BUILD ON CACHE BOOL "")
   set(RUNTIMES_${target}_LIBC_ENABLE_USE_BY_CLANG ON CACHE BOOL "")
+  set(RUNTIMES_${target}_LIBC_USE_NEW_HEADER_GEN OFF CACHE BOOL "")
   set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_CXX_ABI none CACHE STRING "")
   set(RUNTIMES_${target}_LIBCXX_ENABLE_SHARED OFF CACHE BOOL "")