[lldb] [Process/FreeBSDRemote] Introduce powerpc support

Introduce a minimal support for the 32-bit powerpc platform.  This
includes support for GPR and FPR registers.  I also needed to add
software breakpoint opcode for PPC32/PPC64 (big endian), and to fix
offsets in RegisterInfos_powerpc.h (used only by FreeBSD register
context to be globally unique rather than relative to each struct).

Differential Revision: https://reviews.llvm.org/D95947