testsuite: Update Future VSX arithmetic tests

The existing tests use inline asm to force operands into FPRs so that
Future VSX arithmetic instructions are generated.

Remove the operand forcing and let the tests check the compiler's
default instruction selection. This allows the tests to catch any
future changes in instruction selection caused by alternative
reordering.

2026-07-01  Jeevitha Palanisamy  <jeevitha@linux.ibm.com>

gcc/testsuite:
	* gcc.target/powerpc/vsx_arith_builtin-1.c: Remove inline asm
	that forces operands into FPRs.
	* gcc.target/powerpc/vsx_arith_builtin-2.c: Likewise.
	* gcc.target/powerpc/vsx_arith_vect_1.c: Likewiswe.