LoongArch: testsuite: Fix loongarch/vect-frint-scalar.c.

LoongArch: testsuite: Fix loongarch/vect-frint-scalar.c.

In r15-5327, change the default language version for C compilation from
-std=gnu17 to -std=gnu23.

ISO C99 and C11 allow ceil, floor, round and trunc, and their float and
long double variants, to raise the “inexact” exception,
but ISO/IEC TS 18661-1:2014, the C bindings to IEEE 754-2008, as
integrated into ISO C23, does not allow these functions to do so.

So add '-ffp-int-builtin-inexact' to this test case.

gcc/testsuite/ChangeLog:

	* gcc.target/loongarch/vect-frint-scalar.c: Add
	'-ffp-int-builtin-inexact'.