perf: perf can not parser the backtrace of app in the 32bit system and 64bit kernel.

Record PC value from regs[15], it should be regs[32], which cause perf
parser the backtrace failed.

Signed-off-by: Jiping Ma <jiping.ma2@windriver.com>
Signed-off-by: Bruce Ashfield <bruce.ashfield@gmail.com>