[RISCV] Don't force absolute FK_Data_X fixups to relocs

The current behavior of shouldForceRelocation forces relocations for the
majority of fixups when relaxation is enabled. This makes sense for
fixups which incorporate symbols but is unnecessary for simple data
fixups where the fixup target is already resolved to an absolute value.

Differential Revision: https://reviews.llvm.org/D63404
Patch by Edward Jones.

llvm-svn: 369257