[gdb/testsuite] Generate DW_MACRO_define_strp in dwarf assembly

Add support for DW_MACRO_define_strp in dwarf assembly, and use it in
test-case gdb.dwarf2/macro-source-path.exp.

Tested on x86_64-linux.

Approved-By: Tom Tromey <tom@tromey.com>
2 files changed