arc: Use -mcpu=XXX instead of obsolete -marcXXX

With newer ARC tools old way of CPU specification gets obsolete,
so we're switching to newer and more common way of setting "-mcpu".

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/arch/arc/config.mk b/arch/arc/config.mk
index 7c974f0..13676bd 100644
--- a/arch/arc/config.mk
+++ b/arch/arc/config.mk
@@ -31,15 +31,15 @@
 endif
 
 ifdef CONFIG_CPU_ARC750D
-PLATFORM_CPPFLAGS += -marc700
+PLATFORM_CPPFLAGS += -mcpu=arc700
 endif
 
 ifdef CONFIG_CPU_ARC770D
-PLATFORM_CPPFLAGS += -marc700 -mlock -mswape
+PLATFORM_CPPFLAGS += -mcpu=arc700 -mlock -mswape
 endif
 
 ifdef CONFIG_CPU_ARCEM6
-PLATFORM_CPPFLAGS += -marcem
+PLATFORM_CPPFLAGS += -mcpu=arcem
 endif
 
 ifdef CONFIG_CPU_ARCHS34