(WIP) RISC-V: Relax RISCV_PCREL_* to RISCV_GPREL_*

In the medany code model the compiler generates PCREL_HI20+PCREL_LO12
relocation pairs against local symbols because HI20+LO12 relocations
can't reach high addresses.  We relax HI20+LO12 pairs to GPREL
relocations when possible, which is an important optimization for
Dhrystone.  Without this commit we are unable to relax
PCREL_HI20+PCREL_LO12 pairs to GPREL when possible, causing a XX%
permormance hit on Dhrystone on Rocket.

Note that we'll now relax

  la gp, __global_pointer$

to

  mv gp, gp

which probably isn't what you want in your entry code.  Users who want
gp-relative symbols to continue to resolve should add ".option norelax"
accordingly.  Due to this, the assembler now pairs PCREL relocations
with RELAX relocations when they're expected to be relaxed.