aarch64: Ensure ZT0 is zeroed in a new-ZT0 function

aarch64: Ensure ZT0 is zeroed in a new-ZT0 function

ACLE guarantees that a function like:

  __arm_new("zt0") foo() { ... }

will start with ZT0 equal to zero.  I'd forgotten to enforce that
after commiting a lazy save.  After such a save, we should zero
ZA iff the function has ZA state and zero ZT0 iff the function
has ZT0 state.

gcc/
	* config/aarch64/aarch64.cc (aarch64_mode_emit_local_sme_state):
	In the code that commits a lazy save, only zero ZA if the function
	has ZA state.  Similarly zero ZT0 if the function has ZT0 state.

gcc/testsuite/
	* gcc.target/aarch64/sme/zt0_state_5.c (test3): Expect ZT0 rather
	than ZA to be zeroed.
	(test5): Remove zeroing of ZA.