powerpc/boot: Use clang when CC is clang

The gcc compiler may not be available if CC is clang.
Signed-off-by: Bill Wendling <morbo@google.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201120224034.191382-3-morbo@google.com

arch/powerpc/boot/Makefile

@@ -21,7 +21,11 @@
 all: $(obj)/zImage
 
 ifdef CROSS32_COMPILE
+ifdef CONFIG_CC_IS_CLANG
+    BOOTCC := $(CROSS32_COMPILE)clang
+else
     BOOTCC := $(CROSS32_COMPILE)gcc
+endif
     BOOTAR := $(CROSS32_COMPILE)ar
 else
     BOOTCC := $(CC)