gdb/testsuite: make gdb_breakpoint and runto take a linespec

gdb/testsuite: make gdb_breakpoint and runto take a linespec

Change gdb_breakpoint to accept a linespec, not just a function.  In
fact, no behavior changes are necessary, this only changes the parameter
name and documentation.  Change runto as well, since the two are so
close (runto forwards all its arguments to gdb_breakpoint).

I wrote this for a downstrean GDB port,  but thought it could be
useful upstream, eventually, even though not callers take advantage of
it yet.

Change-Id: I08175fd444d5a60df90fd9985e1b5dfd87c027cc