[CMake] Default ENABLE_X86_RELAX_RELOCATIONS to ON

This makes clang default to -Wa,-mrelax-relocations=yes, which enables
R_386_GOT32X (GNU as enables it regardless of -mrelax-relocations=) and
R_X86_64_[REX_]GOTPCRELX in MC. The produced object files require GNU ld>=2.26
to link. binutils 2.26 is considered a very old release today.

Author: MaskRay

clang/CMakeLists.txt

@@ -259,7 +259,7 @@ set(DEFAULT_SYSROOT "" CACHE STRING
 
 set(ENABLE_LINKER_BUILD_ID OFF CACHE BOOL "pass --build-id to ld")
 
-set(ENABLE_X86_RELAX_RELOCATIONS OFF CACHE BOOL
+set(ENABLE_X86_RELAX_RELOCATIONS ON CACHE BOOL
     "enable x86 relax relocations by default")
 
 set(ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER FALSE CACHE BOOL

llvm/utils/gn/secondary/clang/include/clang/Config/BUILD.gn

@@ -33,7 +33,7 @@ write_cmake_config("Config") {
     "GCC_INSTALL_PREFIX=",
     "BACKEND_PACKAGE_STRING=LLVM ${llvm_version}git",
     "ENABLE_LINKER_BUILD_ID=",
-    "ENABLE_X86_RELAX_RELOCATIONS=",
+    "ENABLE_X86_RELAX_RELOCATIONS=1",
     "ENABLE_EXPERIMENTAL_NEW_PASS_MANAGER=",
     "CLANG_ENABLE_OBJC_REWRITER=1",  # FIXME: flag?
     "CLANG_SYSTEMZ_DEFAULT_ARCH=z10",