[Fuchsia] Set ENABLE_X86_RELAX_RELOCATIONS for Fuchsia builds

[mcgrathr]

Summary

This is a "Does your linker support it?" option, and all ours do.

[phosek]

LGTM

cmake/caches/Fuchsia-stage2.cmake

 set(CLANG_PLUGIN_SUPPORT OFF CACHE BOOL "")
 
 set(LLVM_ENABLE_LTO ON CACHE BOOL "")
 if(NOT APPLE)
   set(LLVM_ENABLE_LLD ON CACHE BOOL "")
   set(CLANG_DEFAULT_LINKER lld CACHE STRING "")
 endif()
 
+# This is a "Does your linker support it?" option that only applies
+# to x86-64 ELF targets.  All Fuchsia target linkers do support it.
+# For x86-64 Linux, it's supported by LLD and by GNU linkers since
+# binutils 2.27, so one can hope that all Linux hosts in use handle it.
+# Ideally this would be settable as a per-target option.
+set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL "")
+
 if(APPLE)
   set(LLDB_CODESIGN_IDENTITY "" CACHE STRING "")
 endif()
 
 set(CMAKE_BUILD_TYPE RelWithDebInfo CACHE STRING "")
 set(CMAKE_C_FLAGS_RELWITHDEBINFO "-O2 -gline-tables-only -DNDEBUG" CACHE STRING "")
 set(CMAKE_CXX_FLAGS_RELWITHDEBINFO "-O2 -gline-tables-only -DNDEBUG" CACHE STRING "")