AArch64: gdbserver: read pauth registers

AArch64: gdbserver: read pauth registers

Add the pauth registers to the regset lists.

Add a new regset type OPTIONAL_REGS which allows for the regset read to fail.
Once the read fails, it will not be checked again.  This allows targets with
optional features to keep a single static regset_info structure.

gdb/ChangeLog:

	* arch/aarch64.h (AARCH64_PAUTH_REGS_SIZE): New define.

gdb/gdbserver/ChangeLog:

	* linux-aarch64-low.c (aarch64_store_pauthregset): New function.
	* linux-low.c (regsets_store_inferior_registers): Allow optional reads
	to fail.
	* linux-low.h (enum regset_type): Add OPTIONAL_REGS.