arch: riscv: remove unneeded context switch to gp register

RISC-V global pointer (GP) register is neither caller nor callee
register, and it's a constant value in the single ELF file. Thus, we
don't need to save/restore GP at ISR enter/exit. Remove it to optimize
context switch performance.

Signed-off-by: Jim Shu <cwshu@andestech.com>