libdwarf: add powerpc64 relocation support

Due to missing relocation support in libdwarf for powerpc64, handling of
dwarf info on unlinked objects was bogus.

Examining raw dwarf data on objects compiled on ppc64 with a modern
compiler, one would find that the dwarf data appeared corrupt, with
repeated references to the compiler version where things like types and
function names should appear.

This happenend because the 0 offset of .debug_str contains the compiler
version, and without applying the relocations, *all* indirect strings in
.dwarf_info end up pointing to it.

However, examining the dwarf data on a compiled executable appeared
correct because during final link the relocations get applied and baked
in by the linker.

On FreeBSD this corruption then propogated to CTF data, as ctfconvert
relies on libdwarf to read the dwarf info, for every compiled object
(when building a kernel).

FreeBSD's older in-tree gcc tends to hide the issue, since it only rarely
generates relocations in .debug_info and uses DW_FORM_str instead of
DW_FORM_strp for everything.

Submitted by:	Brandon Bergren, via Justin Hibbits
Obtained from:	FreeBSD r348347

Author: emaste

libdwarf/libdwarf_reloc.c

@@ -44,7 +44,7 @@ _dwarf_get_reloc_type(Dwarf_P_Debug dbg, int is64)
 	case DW_ISA_SPARC:
 		return (is64 ? R_SPARC_UA64 : R_SPARC_UA32);
 	case DW_ISA_PPC:
-		return (R_PPC_ADDR32);
+		return (is64 ? R_PPC64_ADDR64 : R_PPC_ADDR32);
 	case DW_ISA_ARM:
 		return (R_ARM_ABS32);
 	case DW_ISA_MIPS:
@@ -97,6 +97,12 @@ _dwarf_get_reloc_size(Dwarf_Debug dbg, Dwarf_Unsigned rel_type)
 		if (rel_type == R_PPC_ADDR32)
 			return (4);
 		break;
+	case EM_PPC64:
+		if (rel_type == R_PPC_ADDR32)
+			return (4);
+		else if (rel_type == R_PPC64_ADDR64)
+			return (8);
+		break;
 	case EM_MIPS:
 		if (rel_type == R_MIPS_32)
 			return (4);