[lldb] [ABI/AArch64] Add pseudo-regs if missing

Create pseudo-registers on the AArch64 target if they are not provided
by the remote server. This is the case for gdbserver. The created
registers are:

- 32-bit wN partials for 64-bit xN registers
- double precision floating-point dN registers (overlapping with vN)
- single precision floating-point sN registers (overlapping with vN)

Differential Revision: https://reviews.llvm.org/D109876