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.
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