aarch64: Add fallback if ARM_CC_FOR_TARGET not set

aarch64: Add fallback if ARM_CC_FOR_TARGET not set

On Aarch64, you can set ARM_CC_FOR_TARGET to point to the 32-bit
compiler to use when testing gdb.multi/multi-arch.exp and
gdb.multi/multi-arch-exec.exp.  If you don't set it, then those
testcases don't run.

I guess that approximately nobody remembers to set ARM_CC_FOR_TARGET.

This commit adds a fallback.  If ARM_CC_FOR_TARGET is not set, and
testing for Linux, try arm-linux-gnueabi-gcc,
arm-none-linux-gnueabi-gcc, arm-linux-gnueabihf-gcc as 32-bit
compilers, making sure that the produced executable runs on the target
machine before claiming that the compiler produces useful executables.

Change-Id: Iefe5865d5fc84b4032eaff7f4c5c61582bf75c39