New proc is_aarch32_target

New proc is_aarch32_target

GDB tests running on arm target should be also run on aarch32
(32-bit mode on aarch64).  There should be no difference.  It is not
precise to check target triplet to decide which tests should be run,
because if I compiler all the test binary in 32-bit (arm program),
but target triplet is still aarch64, so that these arm specific tests
are skipped.

This patch is to add a new proc is_aarch32_target which return true
if target triplet is arm or the test binary is compiled for arm.

gdb/testsuite:

2015-07-07  Yao Qi  <yao.qi@linaro.org>

	* lib/gdb.exp (is_aarch32_target): New proc.
	* gdb.arch/arm-bl-branch-dest.exp: Check is_aarch32_target
	instead of "istarget "arm*-*-*"".
	* gdb.arch/arm-disp-step.exp: Likewise.
	* gdb.arch/thumb-bx-pc.exp: Likewise.
	* gdb.arch/thumb-prologue.exp: Likewise.
	* gdb.arch/thumb-singlestep.exp: Likewise.
	* gdb.base/disp-step-syscall.exp: Likewise.
	* gdb.base/float.exp: Likewise.