build: Introduce ARM Compiler 6 support

Only the compiler is switched to ARM Compiler 6.  The assembler and linker
are provided by the GCC toolchain.

ARM Compiler 6 is used to build TF when the base name of the path assigned
to `CC` matches the string 'armclang'.

`CROSS_COMPILE` is still needed and should point to the appropriate
GCC toolchain.

Tested with ARM CC 6.7.

Change-Id: Ib359bf9c1e8aeed3f662668e44830864f3fe7b4a
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>