For PPC64: elf_gnu_ifunc_record_cache: handle plt symbols in .text section

elf_gnu_ifunc_record_cache doesn't ever record anything on PPC64
(tested on gcc110 on the compile farm, CentOS 7.4, ELFv1), because
that expects to find PLT symbols in the .plt section, while there we
get:

  (gdb) info symbol 'gnu_ifunc@plt'
  gnu_ifunc@plt in section .text
                           ^^^^^

I guess that may be related to the comment in ppc-linux-tdep.c that
says "For secure PLT, stub is in .text".

In any case, this commit fixes the issue by making the function look
at the symbol name instead of at the section.

gdb/ChangeLog:
yyyy-mm-dd  Pedro Alves  <palves@redhat.com>

	* elfread.c (elf_gnu_ifunc_record_cache): Check if the symbol name
	ends in "@plt" instead of looking at the symbol's section.