[lldb] [Process/Utility] Define dN regs on ARM via helper macro

Use FPU_REG macro to define dN registers, removing the wrong value_regs
while at it.  This is a piece-wise attempt of reconstructing D112066
with the goal of figuring out which part of the larger change breaks
the buildbot.

Differential Revision: https://reviews.llvm.org/D112066