riscv: Provide a definition for 'pause'

If we're going to provide the encoding for 'pause' in cpu_relax()
anyway, then we can drop the toolchain checks and just always use
it. The advantage of doing this is that other code that need
pause don't need to also define it (yes, another use is coming).
Add the definition to insn-def.h since it's an instruction
definition and also because insn-def.h doesn't include much, so
it's safe to include from asm/vdso/processor.h without concern for
circular dependencies.

Signed-off-by: Andrew Jones <ajones@ventanamicro.com>
Link: https://lore.kernel.org/r/20240426100820.14762-9-ajones@ventanamicro.com


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