Tweak gdb.cp tests for aarch32

Tweak gdb.cp tests for aarch32

There are some gdb.cp/ tests fails if the program is compiled for arm
32-bit but GDB/GDBserver is aarch64 64-bit program, because target triplet
doesn't match "arm*-*-*".  Instead, we can use is_aarch32_target.

gdb/testsuite:

2016-08-01  Yao Qi  <yao.qi@linaro.org>

	* gdb.cp/anon-struct.exp: Check is_aarch32_target.
	* gdb.cp/cpexprs.exp: Likewise.
	* gdb.cp/m-static.exp: Likewise.