Fix for prologue processing on PowerPC

One of conditions in skip_prologue() was never visited if there was mflr
instruction that moves the link register to a register different than r0.
This condition expects non shifted value of `lr_reg`. Previously offset
of link register was never saved for registers different than r0.

gdb/ChangeLog:

2018-01-31  Nikola Prica  <nikola.prica@rt-rk.com>

	* rs6000-tdep.c (skip_prologue): Remove shifting for lr_reg and
	assign shifted lr_reg to fdata->lr_register when lr_reg is set.

gdb/testsuite/ChangeLog:

2018-01-31  Nikola Prica  <nikola.prica@rt-rk.com>

	* gdb.arch/powerpc-prologue-frame.s: New file.
	* gdb.arch/powerpc-prologue-frame.c: Likewise.
	* gdb.arch/powerpc-prologue-frame.exp: Likewise.