Fix for prologue processing on PowerPC

[Nikola Prica]

After analyzing dump of ppc program, whose crash occurred after 
watchdog_force_here () function, GDB couldn't print full back trace 
because GDB couldn't unwind PC from the watchdog fucntion.

The problem is introduced with the following patch:

https://sourceware.org/ml/gdb-patches/2008-08/msg00245.html

In function skip_prologue(), shifted lr_reg makes below condition always 
false because non-shifted lr_reg value is expected to be checked.

    else if (lr_reg >= 0 &&
         /* std Rx, NUM(r1) || stdu Rx, NUM(r1) */
         (((op & 0xffff0000) == (lr_reg | 0xf8010000)) ||
         /* stw Rx, NUM(r1) */
         ((op & 0xffff0000) == (lr_reg | 0x90010000)) ||
         /* stwu Rx, NUM(r1) */
         ((op & 0xffff0000) == (lr_reg | 0x94010000))))

Before this fix unwinding was able to work because it relied on unwind
directives or on some of the next frames to find PC. Problem came with
watchdog_force_here() function which didn't contain unwind directives.

I wasn't able to produce test case that would show improvements for end
user. I suppose that changes would be visible if watchdog event was 
called, but I don't have valid ppc board to try this. I have tried this 
code on simple test case with few functions in back trace. The back 
trace is printed correctly with and without this fix, but the difference 
between those two runs is that the body of the upper condition was 
visited with this patch. After visiting the body there was no need to 
look for PC counter in next frames nor to use unwind directives.
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