riscv: implement local_irq_{save,restore} macros

Add implementations of the local_irq_{save,restore} macros so that
<asm/atomic.h> can be used with riscv.

Signed-off-by: Ben Dooks <ben.dooks@sifive.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>