[Fuchsia] Simplified the stage2 build setup

[haowei]

Summary

This patch simplified the BOOTSTRAP_ flags, allowing them to be passed through from regular flags.

[phosek]

LGTM

clang/cmake/caches/Fuchsia.cmake

 set(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR ON CACHE BOOL "")
 set(LLVM_ENABLE_TERMINFO OFF CACHE BOOL "")
 set(LLVM_ENABLE_UNWIND_TABLES OFF CACHE BOOL "")
 set(LLVM_ENABLE_Z3_SOLVER OFF CACHE BOOL "")
 set(LLVM_ENABLE_ZLIB OFF CACHE BOOL "")
 set(LLVM_INCLUDE_DOCS OFF CACHE BOOL "")
 set(LLVM_INCLUDE_EXAMPLES OFF CACHE BOOL "")
 
+# Passthrough stage1 flags to stage1.
+set(_FUCHSIA_BOOTSTRAP_PASSTHROUGH
+  LLVM_ENABLE_ZLIB
+  ZLIB_INCLUDE_DIR
+  ZLIB_LIBRARY
+  LLVM_ENABLE_ZSTD
+  zstd_DIR
+  LLVM_ENABLE_LIBXML2
+  LibXml2_ROOT
+  LLVM_ENABLE_CURL
+  CURL_ROOT
+  OpenSSL_ROOT
+  CMAKE_FIND_PACKAGE_PREFER_CONFIG
+  CMAKE_SYSROOT
+  CMAKE_MODULE_LINKER_FLAGS
+  CMAKE_SHARED_LINKER_FLAGS
+  CMAKE_EXE_LINKER_FLAGS
+)
+
+foreach(variable ${_FUCHSIA_BOOTSTRAP_PASSTHROUGH})
+  get_property(is_value_set CACHE ${variable} PROPERTY VALUE SET)
+  if(${is_value_set})
+    get_property(value CACHE ${variable} PROPERTY VALUE)
+    get_property(type CACHE ${variable} PROPERTY TYPE)
+    set(BOOTSTRAP_${variable} "${value}" CACHE ${type} "")
+  endif()
+endforeach()
+
 if(WIN32)
   set(LLVM_USE_CRT_RELEASE "MT" CACHE STRING "")
 endif()
 
 set(CLANG_DEFAULT_CXX_STDLIB libc++ CACHE STRING "")
 set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
 set(CLANG_DEFAULT_OBJCOPY llvm-objcopy CACHE STRING "")
 set(CLANG_DEFAULT_RTLIB compiler-rt CACHE STRING "")
if(STAGE2_LINUX_${target}_SYSROOT)
     set(RUNTIMES_${target}_LIBCXX_ABI_VERSION 2 CACHE STRING "")
     set(RUNTIMES_${target}_LLVM_ENABLE_ASSERTIONS OFF CACHE BOOL "")
     set(RUNTIMES_${target}_LLVM_ENABLE_RUNTIMES "compiler-rt;libcxx;libcxxabi;libunwind" CACHE STRING "")
     set(RUNTIMES_${target}_SANITIZER_CXX_ABI "libc++" CACHE STRING "")
     set(RUNTIMES_${target}_SANITIZER_CXX_ABI_INTREE ON CACHE BOOL "")
   endif()
 endif()
 
-if(UNIX)
-  set(BOOTSTRAP_CMAKE_SHARED_LINKER_FLAGS "-ldl -lpthread" CACHE STRING "")
-  set(BOOTSTRAP_CMAKE_MODULE_LINKER_FLAGS "-ldl -lpthread" CACHE STRING "")
-  set(BOOTSTRAP_CMAKE_EXE_LINKER_FLAGS "-ldl -lpthread" CACHE STRING "")
-endif()
-
 set(BOOTSTRAP_LLVM_ENABLE_LLD ON CACHE BOOL "")
 set(BOOTSTRAP_LLVM_ENABLE_LTO ON CACHE BOOL "")
 
 set(CLANG_BOOTSTRAP_TARGETS
   check-all
   check-clang
   check-lld
   check-llvm