Fix breakpoints in ifunc after inferior resolved it (@got.plt symbol creation)

Setting a breakpoint on an ifunc symbol after the ifunc has already
been resolved by the inferior should result in creating a breakpoint
location at the ifunc target.  However, that's not what happens on
current Fedora:

  (gdb) n
  53        i = gnu_ifunc (1);    /* break-at-call */
  (gdb)
  54        assert (i == 2);
  (gdb) b gnu_ifunc
  Breakpoint 2 at gnu-indirect-function resolver at 0x7ffff7bd36ee
  (gdb) info breakpoints
  Num     Type                   Disp Enb Address            What
  2       STT_GNU_IFUNC resolver keep y   0x00007ffff7bd36ee <gnu_ifunc+4>

The problem is that elf_gnu_ifunc_resolve_by_got never manages to
resolve an ifunc target.  The reason is that GDB never actually
creates the internal got.plt symbols:

 (gdb) p 'gnu_ifunc@got.plt'
 No symbol "gnu_ifunc@got.plt" in current context.

and this is because GDB expects that rela.plt has relocations for
.plt, while it actually has relocations for .got.plt:

 Relocation section [10] '.rela.plt' for section [22] '.got.plt' at offset 0x570 contains 2 entries:
   Offset              Type            Value               Addend Name
   0x0000000000601018  X86_64_JUMP_SLOT 000000000000000000      +0 __assert_fail
   0x0000000000601020  X86_64_JUMP_SLOT 000000000000000000      +0 gnu_ifunc


Using an older system on the GCC compile farm (machine gcc15, an
x86-64 running Debian 6.0.8, with GNU ld 2.20.1), we see that it used
to be that we'd get a .rela.plt section for .plt:

 Relocation section [ 9] '.rela.plt' for section [11] '.plt' at offset 0x578 contains 3 entries:
   Offset              Type            Value               Addend Name
   0x0000000000600cc0  X86_64_JUMP_SLOT 000000000000000000      +0 __assert_fail
   0x0000000000600cc8  X86_64_JUMP_SLOT 000000000000000000      +0 __libc_start_main
   0x0000000000600cd0  X86_64_JUMP_SLOT 000000000000000000      +0 gnu_ifunc

Those offsets did point into .got.plt, as seen with objdump -h:

  20 .got.plt      00000030  0000000000600ca8  0000000000600ca8  00000ca8  2**3
     		   CONTENTS, ALLOC, LOAD, DATA

I also tested on gcc110 on the compile farm (PPC64 running CentOS
7.4.1708, with GNU ld 2.25.1), and there we see instead:

 Relocation section [ 9] '.rela.plt' for section [23] '.plt' at offset 0x5d0 contains 4 entries:
   Offset              Type            Value               Addend Name
   0x0000000010020148  PPC64_JMP_SLOT  000000000000000000      +0 __libc_start_main
   0x0000000010020160  PPC64_JMP_SLOT  000000000000000000      +0 __gmon_start__
   0x0000000010020178  PPC64_JMP_SLOT  000000000000000000      +0 __assert_fail
   0x0000000010020190  PPC64_JMP_SLOT  000000000000000000      +0 gnu_ifunc

But note that those offsets point into .plt, not .got.plt, as seen
with objdump -h:

 22 .plt          00000078  0000000010020130  0000000010020130  00010130  2**3
                  ALLOC

This commit makes us support all the different combinations above.

With that addressed, we now get:

 (gdb) p 'gnu_ifunc@got.plt'
 $1 = (<text from jump slot in .got.plt, no debug info>) 0x400753 <final>

And setting a breakpoint on the ifunc finds the ifunc target:

 (gdb) b gnu_ifunc
 Breakpoint 2 at 0x400753
 (gdb) info breakpoints
 Num     Type           Disp Enb Address            What
 2       breakpoint     keep y   0x0000000000400753 <final>

gdb/ChangeLog:
2018-04-26  Pedro Alves  <palves@redhat.com>

	* elfread.c (elf_rel_plt_read): Look for relocations for .got.plt too.

1 files changed, 4 insertions, 0 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index a5cb3bb..3266d7f 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2018-04-26  Pedro Alves  <palves@redhat.com>
+
+	* elfread.c (elf_rel_plt_read): Look for relocations for .got.plt too.
+
 2018-04-25  Pedro Alves  <palves@redhat.com>
 
 	* infcmd.c (kill_command): Print the pid as string, not the whole