arm64: add ptrace regsets for ptrauth key management

Add two new ptrace regsets, which can be used to request and change the
pointer authentication keys of a thread. NT_ARM_PACA_KEYS gives access
to the instruction/data address keys, and NT_ARM_PACG_KEYS to the
generic authentication key. The keys are also part of the core dump file
of the process.

The regsets are only exposed if the kernel is compiled with
CONFIG_CHECKPOINT_RESTORE=y, as the only intended use case is
checkpointing and restoring processes that are using pointer
authentication. (This can be changed later if there are other use
cases.)

Reviewed-by: Dave Martin <Dave.Martin@arm.com>
Signed-off-by: Kristina Martsenko <kristina.martsenko@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>

include/uapi/linux/elf.h

@@ -421,6 +421,8 @@ typedef struct elf64_shdr {
 #define NT_ARM_SYSTEM_CALL	0x404	/* ARM system call number */
 #define NT_ARM_SVE	0x405		/* ARM Scalable Vector Extension registers */
 #define NT_ARM_PAC_MASK		0x406	/* ARM pointer authentication code masks */
+#define NT_ARM_PACA_KEYS	0x407	/* ARM pointer authentication address keys */
+#define NT_ARM_PACG_KEYS	0x408	/* ARM pointer authentication generic key */
 #define NT_ARC_V2	0x600		/* ARCv2 accumulator/extra registers */
 #define NT_VMCOREDD	0x700		/* Vmcore Device Dump Note */
 #define NT_MIPS_DSP	0x800		/* MIPS DSP ASE registers */