MIPS/opcodes: Mark descriptive SYNC mnemonics as aliases

MIPS/opcodes: Mark descriptive SYNC mnemonics as aliases

Following the way how descriptive SYNC mnemonics have been defined in
the architecture[1][2] mark them as aliases, so that the generic SYNC
instruction can be alternatively disassembled along with its immediate
operand, as noted in the documents referred.

References:

[1] "MIPS Architecture For Programmers, Volume II-A: The MIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00086,
    Revision 5.04, December 11, 2013, Table 4.7 "Encodings of the
    Bits[10:6] of the SYNC instruction; the SType Field", p. 305

[2] "MIPS Architecture for Programmers, Volume II-B: The microMIPS32
    Instruction Set", MIPS Technologies, Inc., Document Number: MD00582,
    Revision 5.04, January 15, 2014, Table 5.28 "Encodings of the
    Bits[10:6] of the SYNC instruction; the SType Field", p. 481

	opcodes/
	* mips-opc.c (mips_builtin_opcodes): Mark "synciobdma", "syncs",
	"syncw", "syncws", "sync_acquire", "sync_mb", "sync_release",
	"sync_rmb" and "sync_wmb" as aliases.
	* micromips-opc.c (micromips_opcodes): Mark "sync_acquire",
	"sync_mb", "sync_release", "sync_rmb" and "sync_wmb" as aliases.

	gas/
	* testsuite/gas/mips/mips32r2-sync-1.d: New test.
	* testsuite/gas/mips/micromips@mips32r2-sync-1.d: New test.
	* testsuite/gas/mips/mips.exp: Run the new tests.