[RISCV] Define FeatureRelax and shouldForceRelocation for RISCV linker relaxation

1. Deine FeatureRelax to enable/disable linker relaxation.

2. Define shouldForceRelocation to preserve relocation types even if the fixup
   can be resolved when linker relaxation enabled. This is necessary for
   correctness as offsets may change during relaxation.

Differential Revision: https://reviews.llvm.org/D46674

llvm-svn: 332318