gdb/testsuite: introduce dwarf5 option to gdb_compile

gdb/testsuite: introduce dwarf5 option to gdb_compile

A few tests on the testsuite require dwarf5 to work. Up until now, the
way to do this was to explicitly add the command line flag -gdwarf-5.
This isn't very portable, in case a compiler requires a different flag
to emit dwarf5.

This commit adds a new option to gdb_compile that would be able to add
the correct flag (if known) or error out in case we are unable to tell
which flag to use. It also changes the existing tests to use this
general option instead of hard coding -gdwarf-5.

Reviewed-by: Keith Seitz <keiths@redhat.com>
Approved-By: Tom Tromey <tom@tromey.com>