[gdb/testsuite] Fix gdb.dwarf2/dw2-entry-points.exp on ppc64le

[gdb/testsuite] Fix gdb.dwarf2/dw2-entry-points.exp on ppc64le

On ppc64le-linux, I run into:
...
(gdb) bt^M
 #0  0x00000000100006dc in foobar (J=2)^M
 #1  0x000000001000070c in prog ()^M
(gdb) FAIL: gdb.dwarf2/dw2-entry-points.exp: bt foo
...

The test-case attemps to emulate additional entry points of a function, with
function bar having entry points foo and foobar:
...
(gdb) p bar
$1 = {void (int, int)} 0x1000064c <bar>
(gdb) p foo
$2 = {void (int, int)} 0x10000698 <foo>
(gdb) p foobar
$3 = {void (int)} 0x100006d0 <foobar>
...

However, when setting a breakpoint on the entry point foo:
...
(gdb) b foo
Breakpoint 1 at 0x100006dc
...
it ends up in foobar instead of in foo, due to prologue skipping, and
consequently the backtrace show foobar instead foo.

The problem is that the test-case does not emulate an actual prologue at each
entry point.

Fix this by disabling the prologue skipping when setting a breakpoint, using
"break *foo".

Tested on ppc64le-linux and x86_64-linux.

Tested-By: Guinevere Larsen <blarsen@redhat.com>
Approved-By: Ulrich Weigand <Ulrich.Weigand@de.ibm.com>
PR testsuite/31232
Bug: https://sourceware.org/bugzilla/show_bug.cgi?id=31232