[lldb] [ABI/X86] Support combining xmm* and ymm*h regs into ymm*

gdbserver does not expose combined ymm* registers but rather XSAVE-style
split xmm* and ymm*h portions.  Extend value_regs to support combining
multiple registers and use it to create user-friendly ymm* registers
that are combined from split xmm* and ymm*h portions.

Differential Revision: https://reviews.llvm.org/D108937