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Support for ARM Compiler C library.

This commit add support for the ARM Compiler C libary in:
- Kconfig
- libc/armstdc

A new Kconfig symbol is added to allow a toolchain to specify if they
support linking with the minimal C library.
Also the CMake variable `TOOLCHAIN_HAS_NEWLIB` is exported to Kconfig
so that CONFIG_NEWLIB_LIBS can only be enabled if the toolchain has
newlib.

The armclang toolchain selects the CMake scatter file generator and
disables support for the LD linker template which is not supported by
armlink.

For the ARM Compiler C library, a corresponding lib/libc/armstc/ folder
with a minimal implementation to work with the ARM Compiler C library
is added.
Signed-off-by: Torsten Rasmussen <Torsten.Rasmussen@nordicsemi.no>