build: Introduce clang support

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

clang is used to build TF when the base name of the path assigned to
`CC` contains the string 'clang'.

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

Tested with clang 3.9.x and 4.0.x.

Change-Id: I53236d64e3c83ad27fc843bae5fcdae30f2e325e
Signed-off-by: dp-arm <dimitris.papastamos@arm.com>