[powerpc] breakpoint inserted past function end

[powerpc] breakpoint inserted past function end

On powerpc, the prologue scanner reads instruction after instruction,
and just skips instructions that do not affect a frame.  This means
that it does not stop if if finds and unexpected instruction (which
could possibly happen with optimization, I presume). To avoid scanning
too many instructions, it tries to establish an upper limit.

The upper limit is first computed using the debugging (line) info,
but if that fails, it falls back on an arbitrary 100 bytes (or 25
instructions).  The problem is that, if the function is shorter than
those 25 instructions, we run the risk of skipping the entire function
and returning a PC that's outside our function.

In the event where we can find a symbol for a given PC (and therefore
can determine function start and end addresses), but cannot find an
upper limit using skip_prologue_using_sal, then we can at least limit
make sure that the 25 instructions do not put us beyour our function.
If it does, then further reduce the upper-limit to the end of the function.

gdb/ChangeLog:

        * rs6000-tdep.c (rs6000_skip_prologue): Make sure that the prologue
        upper limit address is not greater than the function end address
        when the upper limit could not be computed using the debugging
        info.