AArch64: Fix Diagnostic messaging for LD/ST Exclusive.

AArch64: Fix Diagnostic messaging for LD/ST Exclusive.

A summary of what this patch set fixes:

For instructions

	STXR w0,x2,[x0]
	STLXR w0,x2,[x0]

The warning we emit currently is misleading:

Warning: unpredictable: identical transfer and status registers --`stlxr w0,x2,[x0]'
Warning: unpredictable: identical transfer and status registers --`stxr w0,x2,[x0]'

it ought to be:

Warning: unpredictable: identical base and status registers --`stlxr w0,x2,[x0]'
Warning: unpredictable: identical base and status registers --`stxr w0,x2,[x0]'

For instructions:

	ldaxp x0,x0,[x0]
	ldxp x0,x0,[x0]

The warning we emit is incorrect

Warning: unpredictable: identical transfer and status registers --`ldaxp x0,x0,[x0]'
Warning: unpredictable: identical transfer and status registers --`ldxp x0,x0,[x0]'

it ought to be:

Warning: unpredictable load of register pair -- `ldaxp x0,x0,[x0]'
Warning: unpredictable load of register pair -- `ldxp x0,x0,[x0]'

For instructions

	stlxp   w0, x2, x2, [x0]
	stxp    w0, x2, x2, [x0]

We don't emit any warning when it ought to be:

Warning: unpredictable: identical base and status registers --`stlxp w0,x2,x2,[x0]'
Warning: unpredictable: identical base and status registers --`stxp w0,x2,x2,[x0]'

gas/ChangeLog:

2021-04-09  Tejas Belagod  <tejas.belagod@arm.com>

	* config/tc-aarch64.c (warn_unpredictable_ldst): Clean-up diagnostic messages
	for LD/ST Exclusive instructions.
	* testsuite/gas/aarch64/diagnostic.s: Add a diagnostic test for STLXP.
	* testsuite/gas/aarch64/diagnostic.l: Fix-up test after message clean-up.