linker: lld: riscv: Enable gp relaxation for lld

Unlike GNU ld, lld's gp relaxation is disabled by default and must be
explicitly enabled via `--relax-gp`. Pass this flag to enable gp relaxation
for lld when both linker relaxations and gp usage for RISC-V are enabled.

Signed-off-by: Jonathon Penix <jpenix@quicinc.com>