aarch64: Fix DWARF frame register sizes for predicates

aarch64: Fix DWARF frame register sizes for predicates

Jakub pointed out that __builtin_init_dwarf_reg_size_table
set the size of predicate registers to their current runtime
size when compiled with +sve, but to 8 bytes otherwise.
As explained in the comment, both behaviours are wrong.
Predicates change size with VL and should never need to be
restored during unwinding.

In contrast, the call-saved FP&SIMD frame registers are 8 bytes
(even though the hardware registers are at least 16 bytes) and
the call-clobbered registers have zero size.  A zero size seems
correct for predicates too.

gcc/
	* config/aarch64/aarch64.cc (aarch64_dwarf_frame_reg_mode): New
	function.
	(TARGET_DWARF_FRAME_REG_MODE): Define.

gcc/testsuite/
	* gcc.target/aarch64/dwarf_reg_size_1.c: New test.
	* gcc.target/aarch64/dwarf_reg_size_2.c: Likewise.