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


git-svn-id: https://llvm.org/svn/llvm-project/libunwind/trunk@280099 91177308-0d34-0410-b5e6-96231b3b80d8

include/libunwind.h

@@ -151,8 +151,13 @@ enum {
   UNW_X86_ECX = 1,
   UNW_X86_EDX = 2,
   UNW_X86_EBX = 3,
+#ifdef __FreeBSD__
+  UNW_X86_ESP = 4,
+  UNW_X86_EBP = 5,
+#else
   UNW_X86_EBP = 4,
   UNW_X86_ESP = 5,
+#endif
   UNW_X86_ESI = 6,
   UNW_X86_EDI = 7
 };