RISC-V: Zihintpause: add __builtin_riscv_pause

The Zihintpause extension uses an opcode from the 'fence' opcode range
to add a true hint instruction (i.e. if it is not supported on any
given platform, the 'fence' that is encoded will not enforce any
specific ordering on memory accesses) for entering a low-power state
(e.g. in an idle thread).  We expose this new instruction through a
machine-dependent builtin to allow generating it without a requirement
for any inline assembly.

Given that the encoding of 'pause' is valid (as a 'fence' encoding)
even for processors that do not (yet) support Zihintpause, we make
this builtin available without any further TARGET_* constraints.

gcc/ChangeLog:

	* config/riscv/riscv-builtins.cc (struct riscv_builtin_description):
	add the pause machine-dependent builtin with no result and no
	arguments; mark it as always present (pause is a true hint
	that encodes into a fence-insn, if not supported with the new
	pause semantics).
	* config/riscv/riscv-ftypes.def: Add type for void -> void.
	* config/riscv/riscv.md (riscv_pause): Add risc_pause and
	UNSPECV_PAUSE
	* doc/extend.texi: Document __builtin_riscv_pause.
	* optabs.cc (maybe_gen_insn): Allow nops == 0 (void -> void).

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/builtin_pause.c: New test.