Adjust gdb.multi tests for aarch64

Adjust gdb.multi tests for aarch64

Multi-arch related tests under gdb.multi are to compile programs with
the same compiler but different compiler options (-m64 vs -m32).  However,
different compilers are needed to compile both aarch64 program and
arm (aarch32) program.  This patch is to adjust these test cases to
compile programs in different modes with different compiler.

When we use gcc for arm-linux target, its file name can be different,
arm-linux-gnueabihf-gcc, arm-linux-gnueabi-gcc, or arm-none-linux-gnueabi-gcc,
so I add a variable ARM_CC_FOR_TARGET, so that user can set the name
of gcc for arm-linux target on aarch64, like:

 $ make check RUNTESTFLAGS='ARM_CC_FOR_TARGET=arm-linux-gnueabihf-gcc multi-arch.exp'

gdb/testsuite:

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

	* gdb.multi/multi-arch-exec.exp: Set march1 and march2 to "" if target
	is aarch64.  If target is aarch64, set compiler=${ARM_CC_FOR_TARGET}
	if it exists.
	* gdb.multi/multi-arch.exp: Likewise.