Add CMAKE_OBJCOPY defaults to toolchain files

Specify arm-none-eabi-objcopy as default objcopy binary to use for both
armclang and gcc toolchain files.

The core_platform threadx_demo application depends on CMAKE_OBJCOPY to
mark the RTOS default HardFault_Handler function as weak, to not clash
with the target provided HardFault_Handler. On some systems, especially
x86_64 the host version of objcopy can not handle the Arm format and
linking fails. To avoid this problem, specify a default objcopy binary
to use.

Change-Id: I8504dbbe0803db028eb873e8d82c3e5a21983336
Signed-off-by: Jonny Svärd <jonny.svaerd@arm.com>