tc2: fixed armclang build for RSS

* Restricted CROSS_COMPILE flag for gcc alone since this is not supported for armclang toolchain in RSS
* armclang is expected to be added in path as per TF-M documentation. Mentioned that in comments.

Signed-off-by: Tintu Thomas <tintu.thomas@arm.com>
Change-Id: I7091dd87c77166b133ee7f44487fa94825dd3b1f