gprof support for split debuginfo

Use debug BFD for symbol and line information when a separated
debuginfo file is found, enabling gprof to work correctly with
binaries stripped with "strip -g".  Baby steps toward possible
future debuginfod/dwz support.

gprof/ChangeLog:

2026-04-29  Frank Ch. Eigler  <fche@redhat.com>

        * corefile.c: Add support for separated debuginfo files.
        (core_debug_bfd): New static variable to store separated debug BFD.
        (open_separated_debug_file): New function to locate and open external
        debug files via .gnu_debuglink, .gnu_debugaltlink, or build-id.
        (core_init): Call open_separated_debug_file() after opening the main
        binary to locate external debug info.
        * testsuite/Makefile.am, testsuite/tst-gmon-gprof-l2.sh: Run -l
        test again, but against stripped version of test binary.
        * testsuite/Makefile.in: Regenerated.

Signed-off-by: Frank Ch. Eigler <fche@redhat.com>