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>

Makefile

@@ -126,11 +126,18 @@ OD			:=	${CROSS_COMPILE}objdump
 NM			:=	${CROSS_COMPILE}nm
 PP			:=	${CROSS_COMPILE}gcc -E
 
-ASFLAGS_aarch64		=	-mgeneral-regs-only
-TF_CFLAGS_aarch64	=	-mgeneral-regs-only -mstrict-align
+ifneq ($(findstring clang,$(notdir $(CC))),)
+TF_CFLAGS_aarch32	=	-target armv8a-none-eabi
+TF_CFLAGS_aarch64	=	-target aarch64-elf
+else
+TF_CFLAGS_aarch32	=	-march=armv8-a
+TF_CFLAGS_aarch64	=	-march=armv8-a
+endif
+
+TF_CFLAGS_aarch64	+=	-mgeneral-regs-only -mstrict-align
 
 ASFLAGS_aarch32		=	-march=armv8-a
-TF_CFLAGS_aarch32	=	-march=armv8-a
+ASFLAGS_aarch64		=	-march=armv8-a
 
 CPPFLAGS		=	${DEFINES} ${INCLUDES} -nostdinc		\
 				-Wmissing-include-dirs -Werror

include/lib/aarch32/arch_helpers.h

@@ -36,8 +36,8 @@ static inline u_register_t read_ ## _name(void)				\
  *  systems for GCC versions < 4.6. Above GCC 4.6, both Little Endian and
  *  Big Endian systems generate the right instruction encoding.
  */
-#if !(__GNUC__ > (4) || __GNUC__ == (4) && __GNUC_MINOR__ >= (6))
-#error "GCC 4.6 or above is required to build AArch32 Trusted Firmware"
+#if !(__clang__ || __GNUC__ > (4) || __GNUC__ == (4) && __GNUC_MINOR__ >= (6))
+#error "clang or GCC 4.6 or above is required to build AArch32 Trusted Firmware"
 #endif
 
 #define _DEFINE_COPROCR_WRITE_FUNC_64(_name, coproc, opc1, CRm)		\