gdb/elfread: add debug output for GNU ifunc resolution

gdb/elfread: add debug output for GNU ifunc resolution

Add some debug prints throughout the ifunc resolution code, to be able
to better understand what GDB does.  Add the new "set debug gnu-ifunc"
knob to control it.

Add the debug_prefixed_printf_cond_func macro to implement
gnu_ifunc_debug_printf_func, that takes an explicit function name.  This
is needed to avoid showing "operator()" as the function name in the
debug message.

Here is a sample session with the new debug output enabled.

    (gdb) b the_function
    [gnu-ifunc] elf_gnu_ifunc_resolve_name: resolving name "the_function"
    [gnu-ifunc] elf_gnu_ifunc_resolve_by_cache: resolving "the_function" by cache
    [gnu-ifunc] elf_gnu_ifunc_resolve_by_cache: cache miss for "the_function"
    [gnu-ifunc] elf_gnu_ifunc_resolve_by_got: resolving "the_function" by GOT
    [gnu-ifunc] elf_gnu_ifunc_resolve_by_got: GOT entry "the_function@got.plt" points to 0x7ffff7fb7036
    [gnu-ifunc] elf_gnu_ifunc_record_cache: recording cache entry for "the_function" at 0x7ffff7fb7036
    [gnu-ifunc] elf_gnu_ifunc_record_cache: minimal symbol "the_function@plt" at 0x7ffff7fb7030 does not match addr 0x7ffff7fb7036, not caching
    [gnu-ifunc] elf_gnu_ifunc_resolve_by_got: GOT entry "the_function@got.plt" points to 0x7ffff7fb2036
    [gnu-ifunc] elf_gnu_ifunc_record_cache: recording cache entry for "the_function" at 0x7ffff7fb2036
    [gnu-ifunc] elf_gnu_ifunc_record_cache: minimal symbol "the_function@plt" at 0x7ffff7fb2030 does not match addr 0x7ffff7fb2036, not caching
    [gnu-ifunc] elf_gnu_ifunc_resolve_by_got: failed to resolve "the_function" by GOT
    [gnu-ifunc] elf_gnu_ifunc_resolve_name: failed to resolve name "the_function"
    Breakpoint 2 at gnu-indirect-function resolver at 0x7ffff7fa80e9
    (gdb) c
    Continuing.
    [gnu-ifunc] elf_gnu_ifunc_resolver_stop: stop on resolver for "the_function"
    [gnu-ifunc] elf_gnu_ifunc_resolver_stop: created resolver return breakpoint at 0x7ffff7fd7186
    [gnu-ifunc] elf_gnu_ifunc_resolver_return_stop: stop on resolver return
    [gnu-ifunc] elf_gnu_ifunc_resolver_return_stop: resolver for "the_function" returned resolved address=0x7ffff7fad0e9, resolved pc=0x7ffff7fad0e9
    [gnu-ifunc] elf_gnu_ifunc_record_cache: recording cache entry for "the_function" at 0x7ffff7fad0e9
    [gnu-ifunc] elf_gnu_ifunc_record_cache: cached "the_function" -> 0x7ffff7fad0e9 in objfile /home/simark/build/binutils-gdb/gdb/testsuite/outputs/gdb.base/ifunc-resolver/libimpl.so

    Breakpoint 2, the_function_impl_0 (caller_id=1) at /home/simark/src/binutils-gdb/gdb/testsuite/gdb.base/ifunc-resolver-libimpl.c:25
    25        the_function_last_caller_id = caller_id; /* break-in-impl */

Change-Id: I64f667e3457feaedfe9bb530de58faaf22545fa5
Reviewed-By: Eli Zaretskii <eliz@gnu.org>
Approved-by: Kevin Buettner <kevinb@redhat.com>