selftests: breakpoints: allow to cross-compile for aarch64/arm64

To build breakpoint_test_arm64, ARCH value is only tested for "aarch64".
It covers only the native build because it's computed from uname -m output.
For cross-compilation, ARCH is set to arm64 and prevent to cross-compile
the test.
Fix the test to allow both native and cross-compilation of the test.

Note: glibc is missing several of the TRAP_* constants in the userspace
      definitions. Specifically TRAP_BRANCH and TRAP_HWBKPT.
      See https://sourceware.org/bugzilla/show_bug.cgi?id=21286



Signed-off-by: Fathi Boudra <fathi.boudra@linaro.org>
Tested-by: Naresh Kamboju <naresh.kamboju@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>