When armclang is used to cross-compile anything with the Open IoT SDK,
`components/sanitizers/asan/CMakeLists.txt` emits a message

    ERRORUnsupported compiler

It is because when the aforementioned `CMakeLists.txt` calls

    message(ERROR "Unsupported compiler")

when the compiler is not GCC. But `ERROR` is not a valid message type
(`FATAL_ERROR` is) - anything that's not a type is treated as part of
the message itself.

To fix this, we cannot simply switch to `FATAL_ERROR`, because that
directory is always processed, and making the error fatal will break
all armclang builds even when the sanitizer is not used. So, simply
skip adding the library `iotsdk-sanitizers-asan` when GCC is not used.

Signed-off-by: Lingkai Dong <lingkai.dong@arm.com>