fix(build): fix arch32 build issue for clang

Fixed the qemu 32 bit clang build fail caused because of
no march32 directives in TF_CFLAGS_aarch32 variable

march32_directive is initialized later in Makefile and since clang build
uses Immediate set instead of Lazy set , TF_CFLAGS_aarch32 doesn't
have mcpu variable.

Signed-off-by: Arvind Ram Prakash <arvind.ramprakash@arm.com>
Change-Id: I09094a0912ee2d9d0e11f65135a352de8a135936
diff --git a/Makefile b/Makefile
index 8adc0d0..9dfd75b 100644
--- a/Makefile
+++ b/Makefile
@@ -274,7 +274,7 @@
 		TF_CFLAGS_aarch64	:=	-target aarch64-arm-none-eabi $(march64-directive)
 		LD			:=	$(LINKER)
 	else
-		TF_CFLAGS_aarch32	:=	$(target32-directive) $(march32-directive)
+		TF_CFLAGS_aarch32	=	$(target32-directive) $(march32-directive)
 		TF_CFLAGS_aarch64	:=	-target aarch64-elf $(march64-directive)
 		LD			:=	$(shell $(CC) --print-prog-name ld.lld)