toolchain: Add macro for keeping a symbol in the binary

Add LINKER_KEEP macro which can be used to prevent linker garbage
collector from removing unused symbol (function or variable).

Example use case is to use it for functions which are part of a test
application which has functions which are indirectly called by the
host test tools. Since they are not called explicitely by the
application so without marking them with LINKER_KEEP they would be
removed by the linker garbage collector.

Signed-off-by: Krzysztof Chruscinski <krzysztof.chruscinski@nordicsemi.no>