RISC-V: __builtin_riscv_pause for all environment

The "pause" RISC-V hint instruction requires the 'Zihintpause' extension (in
the assembler).  However, GCC emits "pause" unconditionally, making an
assembler error while compiling code with __builtin_riscv_pause while the
'Zihintpause' extension disabled.

However, the "pause" instruction code (0x0100000f) is a HINT and emitting its
instruction code is safe in any environment.

This commit implements handling for the 'Zihintpause' extension and emits
".insn 0x0100000f" instead of "pause" only if the extension is disabled (making
the diagnostics better).

gcc/ChangeLog:

	* common/config/riscv/riscv-common.cc (riscv_ext_version_table):
	Implement the 'Zihintpause' extension, version 2.0.
	(riscv_ext_flag_table) Add 'Zihintpause' handling.
	* config/riscv/riscv-builtins.cc: Remove availability predicate
	"always" and add "hint_pause".
	(riscv_builtins) : Add "pause" extension.
	* config/riscv/riscv-opts.h (MASK_ZIHINTPAUSE, TARGET_ZIHINTPAUSE): New.
	* config/riscv/riscv.md (riscv_pause): Adjust output based on
	TARGET_ZIHINTPAUSE.

gcc/testsuite/ChangeLog:

	* gcc.target/riscv/builtin_pause.c: Removed.
	* gcc.target/riscv/zihintpause-1.c: New test when the 'Zihintpause'
	extension is enabled.
	* gcc.target/riscv/zihintpause-2.c: Likewise.
	* gcc.target/riscv/zihintpause-noarch.c: New test when the 'Zihintpause'
	extension is disabled.

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