[gdb/testsuite] Add gdb.base/fission-macro.exp

Starting with gcc commit 80048aa13a6 ("debug/111409 - don't generate COMDAT
macro sections for split DWARF"), available from release gcc 14.1 onwards, gcc
produces a usable dwarf-5 32-bit .debug_macro.dwo section.

Add a test-case excercising this.

Tested on x86_64-linux.

Tested test-case using a current gcc trunk build, and gcc 14.