testsuite: Compile gcc.dg/tree-ssa/ldexp.c with -std=c99

testsuite: Compile gcc.dg/tree-ssa/ldexp.c with -std=c99

The gcc.dg/tree-ssa/ldexp.c test FAILs on Solaris:

FAIL: gcc.dg/tree-ssa/ldexp.c (test for excess errors)
UNRESOLVED: gcc.dg/tree-ssa/ldexp.c scan-assembler-not \\tfmul\\t

Excess errors:
../builtins-config.h:53:2: error: #error forgot to set -std=c99.

Fixed by adding that option.

Tested on i386-pc-solaris2.11, sparc-sun-solaris2.11, and
x86_64-pc-linux-gnu.

2026-02-16  Rainer Orth  <ro@CeBiTec.Uni-Bielefeld.DE>

	gcc/testsuite:
	* gcc.dg/tree-ssa/ldexp.c (dg-additional-options): Add -std=c99.