trusted-firmware-a: enable clang support

The CC and LD parameters are used to point make to the currently
used compiler and linkers.

This change enables TFA to be compiled successfully when
TOOLCHAIN=clang.

Change-Id: Ice465318b8c9b2b69c08fe4f48f6d36c610978f0
Signed-off-by: Brett Warren <brett.warren@arm.com>
Signed-off-by: Jon Mason <jon.mason@arm.com>