Work around relocation R_X86_64_PC32 link error

A more complete discussion of the error and its background can be found
in SR-1023.

The brief overview is that in versions of `ld` that would be effected by
this dynamic relocation issue there are two flags: `-z nocopyreloc` and
`-z noextern-protected-data`. If `ld` supports the flags then they are
sento to the linker. This patch checks that the linker supports the
flags and then optionally sets them.

The code to check if `ld` supports the flags is inspired by this CMake
mailing list entry.

https://cmake.org/pipermail/cmake/2011-July/045525.html

Author: RLovelett

cmake/modules/AddSwift.cmake

@@ -801,6 +801,19 @@ function(_add_swift_library_single target name)
     list(APPEND link_flags "-fuse-ld=gold")
   endif()
 
+  CMAKE_MINIMUM_REQUIRED(VERSION 2.8 FATAL_ERROR)
+  INCLUDE(CheckCXXCompilerFlag)
+  SET(CMAKE_REQUIRED_FLAGS "-Wl,-z,nocopyreloc")
+  CHECK_CXX_COMPILER_FLAG("" LINKER_SUPPORTS_NOCOPYRELOC)
+  IF(LINKER_SUPPORTS_NOCOPYRELOC)
+      list(APPEND link_flags ${CMAKE_REQUIRED_FLAGS})
+  ENDIF()
+  SET(CMAKE_REQUIRED_FLAGS "-Wl,-z,noextern-protected-data")
+  CHECK_CXX_COMPILER_FLAG("" LINKER_SUPPORTS_NOEXTERN_PROTECTED_DATA)
+  IF(LINKER_SUPPORTS_NOEXTERN_PROTECTED_DATA)
+      list(APPEND link_flags ${CMAKE_REQUIRED_FLAGS})
+  ENDIF()
+
   # Configure plist creation for OS X.
   set(PLIST_INFO_PLIST "Info.plist" CACHE STRING "Plist name")
   if(APPLE AND SWIFTLIB_SINGLE_IS_STDLIB)
@@ -1416,6 +1429,18 @@ function(_add_swift_executable_single name)
     list(APPEND link_flags "-fuse-ld=gold")
   endif()
 
+  INCLUDE(CheckCXXCompilerFlag)
+  SET(CMAKE_REQUIRED_FLAGS "-Wl,-z,nocopyreloc")
+  CHECK_CXX_COMPILER_FLAG("" LINKER_SUPPORTS_NOCOPYRELOC)
+  IF(LINKER_SUPPORTS_NOCOPYRELOC)
+      list(APPEND link_flags ${CMAKE_REQUIRED_FLAGS})
+  ENDIF()
+  SET(CMAKE_REQUIRED_FLAGS "-Wl,-z,noextern-protected-data")
+  CHECK_CXX_COMPILER_FLAG("" LINKER_SUPPORTS_NOEXTERN_PROTECTED_DATA)
+  IF(LINKER_SUPPORTS_NOEXTERN_PROTECTED_DATA)
+      list(APPEND link_flags ${CMAKE_REQUIRED_FLAGS})
+  ENDIF()
+
   # Find the names of dependency library targets.
   #
   # We don't add the ${ARCH} to the target suffix because we want to link