Fix debugging of 32bits arm binaries on arm64.

In set_regs32()/fill_regs32(), we have to get/set SP and LR from/to
tf_x[13] and tf_x[14].
set_regs() and fill_regs() may be called for a 32bits process, if the process
is ptrace'd from a 64bits debugger. So, in set_regs() and fill_regs(), get
or set PC and SPSR from where the debugger expects it, from tf_x[15] and
tf_x[16].