gdb: use gdb_test_multiple in gdb_breakpoint

When running the testsuite in a non-optimized build on a slow machine, I
sometimes get:

    UNTESTED: gdb.gdb/selftest.exp: Cannot set breakpoint at captured_main, skipping testcase.

do_self_tests, in lib/selftest-support.exp, uses `with_timeout_factor
10`, to account for the fact that reading the debug info of the gdb
binary (especially in a non-optimized GDB) can take time.  But then it
ends up calling gdb_breakpoint, which uses gdb_expect with a hard-coded
timeout of 30 seconds.

Fix this by making gdb_breakpoint use gdb_test_multiple, which is a
desired change anyway for this kind of simple command / expected
output case.

Change-Id: I9b06ce991cc584810d8cc231b2b4893980b8be75
Reviewed-By: Lancelot Six <lancelot.six@amd.com>