[PATCH] Fix show_regs to give a backtrace

Currently, show_regs will not give a backtrace unless frame pointer is enabled.
show_regs calls __backtrace which does not support the arm unwind tables and
returns nothing if CONFIG_FRAME_POINTER is not enabled. Having a useful
backtrace when dumping registers is helpful, so fix show_regs to call into
dump_backtrace which properly handles both the CONFIG_FRAME_POINTER and the
CONFIG_ARM_UNWIND case for backtraces.