[gdb/breakpoint] Handle .plt.sec in in_plt_section

[gdb/breakpoint] Handle .plt.sec in in_plt_section

Consider the following test-case small.c:
...
 #include <stdio.h>
 #include <stdlib.h>
 #include <string.h>

 int main (void) {
   int *p = (int *)malloc (sizeof(int) * 4);
   memset (p, 0, sizeof(p));
   printf ("p[0] = %d; p[3] = %d\n", p[0], p[3]);
   return 0;
 }
...

On Ubuntu 20.04, we get:
...
$ gcc -O0 -g small.c
$ gdb -batch a.out -ex start -ex step
Temporary breakpoint 1, main () at small.c:6
6         int *p = (int *) malloc(sizeof(int) * 4);
p[0] = 0; p[3] = 0
[Inferior 1 (process $dec) exited normally]
...
but after switching off the on-by-default fcf-protection, we get the desired
behaviour:
...
$ gcc -O0 -g small.c -fcf-protection=none
$ gdb -batch a.out -ex start -ex step
Temporary breakpoint 1, main () at small.c:6
6         int *p = (int *) malloc(sizeof(int) * 4);
7         memset (p, 0, sizeof(p));
...

Using "set debug infrun 1", the first observable difference between the two
debug sessions is that with -fcf-protection=none we get:
...
[infrun] process_event_stop_test: stepped into dynsym resolve code
...
In this case, "in_solib_dynsym_resolve_code (malloc@plt)" returns true because
"in_plt_section (malloc@plt)" returns true.

With -fcf-protection=full, "in_solib_dynsym_resolve_code (malloc@plt)" returns
false because "in_plt_section (malloc@plt)" returns false, because the section
name for malloc@plt is .plt.sec instead of .plt, which is not handled in
in_plt_section:
...
static inline int
in_plt_section (CORE_ADDR pc)
{
  return pc_in_section (pc, ".plt");
}
...

Fix this by handling .plt.sec in in_plt_section.

Tested on x86_64-linux.

[ Another requirement to be able to reproduce this is to have a dynamic linker
with a "malloc" minimal symbol, which causes find_solib_trampoline_target to
find it, such that skip_language_trampoline returns the address for the
dynamic linkers malloc.  This causes the step machinery to set a breakpoint
there, and to continue, expecting to hit it.  Obviously, we execute glibc's
malloc instead, so the breakpoint is not hit and we continue to program
completion. ]

gdb/ChangeLog:

2021-01-14  Tom de Vries  <tdevries@suse.de>

	PR breakpoints/27151
	* objfiles.h (in_plt_section): Handle .plt.sec.