MIPS/GCC/testsuite: Fix data-sym-pool.c for SVR4 model at -O0

MIPS/GCC/testsuite: Fix data-sym-pool.c for SVR4 model at -O0

With GCC configurations using the SVR4 rather than the PLT dynamic
executable model and the o32 ABI with the data-sym-pool.c test case code
like below is produced:

	.file	1 "data-sym-pool.c"
	.section .mdebug.abi32
	.previous
	.nan	legacy
	.module	fp=xx
	.module	nooddspreg
	.abicalls
	.text
	.align	2
	.globl	frob
	.set	mips16
	.set	nomicromips
	.ent	frob
	.type	frob, @function
frob:
	.frame	$17,8,$31		# vars= 0, regs= 1/0, args= 0, gp= 0
	.mask	0x00020000,-4
	.fmask	0x00000000,0
	save	8,$17
	move	$17,$sp
	lw	$2,$L4
	move	$sp,$17
	restore	8,$17
	jr	$31
	.type	__pool_frob_3, @object
__pool_frob_3:
	.align	2
$L3:
	.word	__gnu_local_gp
$L4:
	.word	305419896
	.type	__pend_frob_3, @function
__pend_frob_3:
	.insn
	.end	frob
	.size	frob, .-frob
	.ident	"GCC: (GNU) 8.0.1 20180410 (experimental)"

causing a failure due to the unexpected `__gnu_local_gp' entry in the
constant pool, even though there is nothing wrong with it as far as the
annotation being examined is concerned.

Given that the SVR4 vs PLT code model consideration is irrelevant for
this test case rather than rewriting the regular expression to match
this variant of code just enforce the PLT model by using the `-mplt'
option.  It is safe to use this option unconditionally as it is silently
ignored with configurations that do not support this model, e.g. bare
metal ELF.

	gcc/testsuite/
	* gcc.target/mips/data-sym-pool.c (dg-options): Add `-mplt'.

From-SVN: r259690