llvm-libunwind: use conventional (non-Darwin) X86 register numbers

For historical reasons Darwin/i386 has ebp and esp swapped in the
eh_frame register numbering. That is:

         Darwin      Other
Reg #    eh_frame    eh_frame    DWARF
=====    ========    ========    =====
  4        ebp         esp        esp
  5        esp         ebp        ebp

Although the UNW_X86_* constants are not supposed to be coupled to
DWARF / eh_frame numbering they are currently conflated in LLVM
libunwind, and thus we require the non-Darwin numbering.

MFC after: 3 days
Sponsored by: The FreeBSD Foundation