[PowerPC] 32-bit ELF PIC support

This adds initial support for PPC32 ELF PIC (Position Independent Code; the
-fPIC variety), thus rectifying a long-standing deficiency in the PowerPC
backend.

Patch by Justin Hibbits!

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@213427 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/Support/ELF.h

@@ -458,6 +458,7 @@ enum {
   R_PPC_GOT16_LO              = 15,
   R_PPC_GOT16_HI              = 16,
   R_PPC_GOT16_HA              = 17,
+  R_PPC_PLTREL24              = 18,
   R_PPC_REL32                 = 26,
   R_PPC_TLS                   = 67,
   R_PPC_DTPMOD32              = 68,