Adding support for the R_X86_64_REX_GOTPCRELX relocation type. 
This relocation is treated by the linker the same as the R_X86_64_GOTPCRELX type
G + GOT + A - P to generate relative offsets to the GOT.
The REX prefix has no influence in this stage.

This caused breakage when enabling relro/bindnow hardening e.g. in ghcPaclages.vector

Source: https://phabricator.haskell.org/D2303#67070
diff --git a/rts/Linker.c b/rts/Linker.c
--- a/rts/Linker.c
+++ b/rts/Linker.c
@@ -5681,7 +5681,13 @@
           *(Elf64_Sword *)P = (Elf64_Sword)value;
 #endif
           break;
-
+/* These two relocations were introduced in glibc 2.23 and binutils 2.26.
+    But in order to use them the system which compiles the bindist for GHC needs
+    to have glibc >= 2.23. So only use them if they're defined. */
+#if defined(R_X86_64_REX_GOTPCRELX) && defined(R_X86_64_GOTPCRELX)
+      case R_X86_64_REX_GOTPCRELX:
+      case R_X86_64_GOTPCRELX:
+#endif
       case R_X86_64_GOTPCREL:
       {
           StgInt64 gotAddress = (StgInt64) &makeSymbolExtra(oc, ELF_R_SYM(info), S)->addr;