readelf: Fix printing of DW_FORM_strx and DW_MACRO parsing

readelf: Fix printing of DW_FORM_strx and DW_MACRO parsing

print_form_data didn't take the offset_len (4 or 8 bytes) into account
causing the wrong entry to be read from .debug_str_offsets.
print_debug_macro_section did sanity checking before calling
print_form_data, which does sanity checking itself. The sanity check
for DW_FORM_strx was wrong in print_debug_macro_section (but correct
in print_form_data).

Add a new testfile for run-readelf-macro.sh, this one compiled with
clang -gdwarf-5 -fdebug-macro.

      * src/readelf.c (print_form_data): Multiply by offset_len for
      strx_val.
      (print_debug_macro_section): Remove sanity checks before calling
      print_form_data.
      * tests/testfileclangmacro.bz2: New testfile.
      * tests/Makefile.am (EXTRA_DIST): Add testfileclangmacro.bz2.
      * tests/run-readelf-macro.sh: Add testfileclangmacro output.

Signed-off-by: Mark Wielaard <mark@klomp.org>