libunwind: fix X86 register numbers for FreeBSD/i386

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

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

Although the UNW_X86_* constants are not intended to be coupled with
DWARF / eh_frame numbering they are currently conflated in libunwind.

Differential Revision:	https://reviews.llvm.org/D22508

llvm-svn: 280099