[PATCH, rs6000, v3][PR gdb/27525] displaced stepping across addpcis/lnia.

  This addresses PR gdb/27525.     The lnia and other variations
of the addpcis instruction write the value of the NIA into a target register.
If we are single-stepping across a breakpoint, the instruction is executed
from a displaced location, and thusly the written value of the PC/NIA
will be incorrect.   The changes here will measure the displacement
offset, and adjust the target register value to compensate.

YYYY-MM-DD  Will Schmidt  <will_schmidt@vnet.ibm.com>

gdb/ChangeLog:

        * rs6000-tdep.c (ppc_displaced_step_fixup): Update to handle
        the addpcis/lnia instruction.

gdb/testsuite/ChangeLog:

        * gdb.arch/powerpc-addpcis.exp: Testcase harness to
        exercise single-stepping over subpcis,lnia,addpcis instructions
        with displacement.
        * gdb.arch/powerpc-addpcis.s: Testcase with stream
        of addpcis/lnia/subpcis instructions.
        * gdb.arch/powerpc-lnia.exp: Testcase harness to exercise
        single-stepping over lnia instructions with displacement.
        * gdb.arch/powerpc-lnia.s: Testcase with stream of
        lnia instructions.