AMDGPU/SI: Don't use fixup_si_rodata for scratch rsrc relocations

Summary:
We need to set the fixup type to FK_Data_4 for the
SCRATCH_RSRC_DWORD[01] symbols, since these require absolute
relocations, and fixup_si_rodata is for relative relocations.

Reviewers: arsenm, kzhuravl

Subscribers: arsenm, kzhuravl, llvm-commits

Differential Revision: http://reviews.llvm.org/D21153

llvm-svn: 272417