readelf: Handle multiple .debug_macro sections and decode header flag.

readelf: Handle multiple .debug_macro sections and decode header flag.

In object files there could be multiple .debug_macro sections.
These are COMDAT sections used as imports. Note that the output for
DW_MACRO_import isn't ideal since the offset is printed against the
start of the .debug_macro section, but it doesn't show which one.
We currently don't have that information and no interface yet for
libdw users.

Also decode the macro header flag byte for convenience.

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