i386: Prefer remote atomic insn for atomic_fetch{add, and, or, xor}

i386: Prefer remote atomic insn for atomic_fetch{add, and, or, xor}

Add flag -mprefer-remote-atomic to control whether to generate raoint
insn for atomic operations.

gcc/ChangeLog:

	* config/i386/i386.opt:Add -mprefer-remote-atomic.
	* config/i386/sync.md (atomic_<plus_logic><mode>):
	New define_expand.
	(atomic_add<mode>): Rename to below one.
	(atomic_add<mode>_1): To this.
	(atomic_<logic><mode>): Ditto.
	(atomic_<logic><mode>_1): Ditto.
	* doc/invoke.texi: Add -mprefer-remote-atomic.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/raoint-atomic-fetch.c: New test.