PR target/103069: Relax cmpxchg loop for x86 target

From the CPU's point of view, getting a cache line for writing is more
expensive than reading.  See Appendix A.2 Spinlock in:

https://www.intel.com/content/dam/www/public/us/en/documents/white-papers/
xeon-lock-scaling-analysis-paper.pdf

The full compare and swap will grab the cache line exclusive and causes
excessive cache line bouncing.

The atomic_fetch_{or,xor,and,nand} builtins generates cmpxchg loop under
-march=x86-64 like:

	movl	v(%rip), %eax
.L2:
	movl	%eax, %ecx
	movl	%eax, %edx
	orl	$1, %ecx
	lock cmpxchgl	%ecx, v(%rip)
	jne	.L2
	movl	%edx, %eax
	andl	$1, %eax
	ret

To relax above loop, GCC should first emit a normal load, check and jump to
.L2 if cmpxchgl may fail. Before jump to .L2, PAUSE should be inserted to
yield the CPU to another hyperthread and to save power, so the code is
like

.L84:
        movl    (%rdi), %ecx
        movl    %eax, %edx
        orl     %esi, %edx
        cmpl    %eax, %ecx
        jne     .L82
        lock cmpxchgl   %edx, (%rdi)
        jne     .L84
.L82:
        rep nop
        jmp     .L84

This patch adds corresponding atomic_fetch_op expanders to insert load/
compare and pause for all the atomic logic fetch builtins. Add flag
-mrelax-cmpxchg-loop to control whether to generate relaxed loop.

gcc/ChangeLog:

	PR target/103069
	* config/i386/i386-expand.c (ix86_expand_atomic_fetch_op_loop):
	New expand function.
	* config/i386/i386-options.c (ix86_target_string): Add
	-mrelax-cmpxchg-loop flag.
	(ix86_valid_target_attribute_inner_p): Likewise.
	* config/i386/i386-protos.h (ix86_expand_atomic_fetch_op_loop):
	New expand function prototype.
	* config/i386/i386.opt: Add -mrelax-cmpxchg-loop.
	* config/i386/sync.md (atomic_fetch_<logic><mode>): New expander
	for SI,HI,QI modes.
	(atomic_<logic>_fetch<mode>): Likewise.
	(atomic_fetch_nand<mode>): Likewise.
	(atomic_nand_fetch<mode>): Likewise.
	(atomic_fetch_<logic><mode>): New expander for DI,TI modes.
	(atomic_<logic>_fetch<mode>): Likewise.
	(atomic_fetch_nand<mode>): Likewise.
	(atomic_nand_fetch<mode>): Likewise.
	* doc/invoke.texi: Document -mrelax-cmpxchg-loop.

gcc/testsuite/ChangeLog:

	PR target/103069
	* gcc.target/i386/pr103069-1.c: New test.
	* gcc.target/i386/pr103069-2.c: Ditto.