[gdb/testsuite] Avoid breakpoint in GLIBC in gdb.threads/execl.exp

[gdb/testsuite] Avoid breakpoint in GLIBC in gdb.threads/execl.exp

When running the gdb.threads/execl.exp test-case, we run into this FAIL:
...
(gdb) continue^M
Continuing.^M
^M
Thread 1 "execl" hit Breakpoint 2, __GI_execl (path=0x6024a0 \
  "build/gdb/testsuite/outputs/gdb.threads/execl/execl1", \
  arg=<optimized out>) at execl.c:51^M
51        if (execl (new_image, new_image, NULL) == -1) \
  /* set breakpoint here */^M
(gdb) FAIL: gdb.threads/execl.exp: continue across exec
...
The fail is due to the continue command hitting a breakpoint in __GI_execl
rather than main.

This problem originates from where we execute the "b 51" command, and get two
breakpoint locations:
...
(gdb) run ^M
Starting program: build/gdb/testsuite/outputs/gdb.threads/execl/execl ^M
[Thread debugging using libthread_db enabled]^M
Using host libthread_db library "/lib64/libthread_db.so.1".^M
^M
Breakpoint 1, main (argc=1, argv=0x7fffffffd3f8) at gdb.threads/execl.c:44^M
44        pthread_create (&thread1, NULL, thread_function, NULL);^M
(gdb) b 51^M
Breakpoint 2 at 0x400787: gdb.threads/execl.c:51. (2 locations)^M
(gdb) PASS: gdb.threads/execl.exp: set breakpoint at execl
...

Adding a "info breakpoints" command, we can see the locations:
...
(gdb) info breakpoints^M
Num     Type           Disp Enb Address            What^M
1       breakpoint     keep y   0x00000000004006ee in main at \
  gdb.threads/execl.c:44^M
        breakpoint already hit 1 time^M
2       breakpoint     keep y   <MULTIPLE>         ^M
2.1                         y   0x0000000000400787 in main at \
  gdb.threads/execl.c:51^M
2.2                         y   0x00007ffff758d925 in __GI_execl at \
  execl.c:51^M
(gdb) PASS: gdb.threads/execl.exp: info breakpoints
...

The fact that the __GI_execl breakpoint location is there, is a bug, filed as
PR25656.  Without debug info for GLIBC though, the bug is not triggered.

Fix the FAIL by working around the bug, and deleting the breakpoint after
hitting the first breakpoint location.

Tested on x86_64-linux.

gdb/testsuite/ChangeLog:

2020-03-12  Tom de Vries  <tdevries@suse.de>

	* gdb.threads/execl.exp: Delete breakpoint after hitting it.