Bring back r323297.

It was reverted because it broke the grub build. The reason the grub
build broke is because grub does its own relocation processing and was
not handing R_386_PLT32. Since grub has no dynamic linker, the fix is
trivial: handle R_386_PLT32 exactly like R_386_PC32.

On the report it was noted that they are using
-fno-integrated-assembler. The upstream GAS (starting with
451875b4f976a527395e9303224c7881b65e12ed) will already be producing a
R_386_PLT32 anyway, so they have to update their code one way or the
other

Original message:

Don't assume a null GV is local for ELF and MachO.

This is already a simplification, and should help with avoiding a plt
reference when calling an intrinsic with -fno-plt.

With this change we return false for null GVs, so the caller only
needs to check the new metadata to decide if it should use foo@plt or
*foo@got.

llvm-svn: 325514