checkstack: add riscv support for scripts/checkstack.pl

scripts/checkstack.pl lacks support for the riscv architecture. Add
support to detect "addi sp,sp,-FRAME_SIZE" stack frame generation instruction

Signed-off-by: Wadim Mueller <wafgo01@gmail.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>