Keep reserved bits in CPSR on write

Keep reserved bits in CPSR on write

In patch https://sourceware.org/ml/gdb-patches/2016-04/msg00529.html
I cleared reserved bits when reading CPSR.  It makes a problem that
these bits (zero) are written back to kernel through ptrace, and it
changes the state of the processor on some recent kernel, which is
unexpected.

In this patch, I keep these reserved bits when write CPSR back to
hardware.

gdb:

2016-09-21  Yao Qi  <yao.qi@linaro.org>

	* aarch32-linux-nat.c (aarch32_gp_regcache_collect): Keep
	bits 20 to 23.

gdb/gdbserver:

2016-09-21  Yao Qi  <yao.qi@linaro.org>

	* linux-aarch32-low.c (arm_fill_gregset): Keep bits 20 to
	23.