Single register access - Target methods and GDB server support

[gsmcmullin]

• Add new target methods to access single registers
• Add GDB packets support for p and P packets
• Implement new methods in Cortex-M and Cortex-A drivers

[UweBonnes]

Where is this access used?

[gsmcmullin]

It's used when GDB sends us one of these packets. It's not needed for the ARM targets, but is needed by upcoming RISC-V support where GDB queries high numbered registers that aren't present in the g and G packets. I might argue that what's being done in RISC-V GDB is bad, but they're unlikely to care.

[UweBonnes]

Perhaps helpful for those thinking about RiscV support