rs6000: Don't ICE when compiling the __builtin_vec_xst_trunc built-in [PR109178]

When we expand the __builtin_vec_xst_trunc built-in, we use the wrong mode
for the MEM operand which causes an unrecognizable insn ICE.  The solution
is to use the correct TMODE mode.

2023-03-20  Peter Bergner  <bergner@linux.ibm.com>

gcc/
	PR target/109178
	* config/rs6000/rs6000-builtin.cc (stv_expand_builtin): Use tmode.

gcc/testsuite/
	PR target/109178
	* gcc.target/powerpc/pr109178.c: New test.