riscv: introduce nops and __nops macros for NOP sequences

NOP sequences tend to get used for padding out alternative sections

This change adds macros for generating these sequences as both inline
asm blocks, but also as strings suitable for embedding in other asm
blocks directly.

It essentially mimics similar functionality from arm64 introduced by
Wil Deacon in commit f99a250c ("arm64: barriers: introduce nops
and __nops macros for NOP sequences").

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20220607143059.1054074-2-heiko@sntech.de


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