riscv: entry: Consolidate general regs saving/restoring

Consolidate the saving/restoring GPs (except zero, ra, sp, gp,
tp and t0) into save_from_x6_to_x31/restore_from_x6_to_x31 macros.

No functional change intended.

Signed-off-by: Jisheng Zhang <jszhang@kernel.org>
Reviewed-by: Guo Ren <guoren@kernel.org>
Reviewed-by: Björn Töpel <bjorn@rivosinc.com>
Tested-by: Guo Ren <guoren@kernel.org>
Signed-off-by: Guo Ren <guoren@kernel.org>
Link: https://lore.kernel.org/r/20230222033021.983168-8-guoren@kernel.org


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>