libdw: Handle other string forms in dwarf_macro_param2

libdw: Handle other string forms in dwarf_macro_param2

dwarf_getmacros handles the additional macro string forms added by DWARF
5, but dwarf_macro_param2 doesn't.  Update it with the list of all
string forms allowed in .debug_macro.  In particular, GCC and Clang
generate DW_MACRO_define_strx and DW_MACRO_undef_strx, which
dwarf_macro_param2 couldn't handle.

Fixes: cdf865b890c2 ("readelf, libdw: Handle DWARF5 .debug_macro.")
Signed-off-by: Omar Sandoval <osandov@fb.com>