ARC: rework setting of ARC CPU specific compiler options

It's a very rare if at all existing occasion when ARC CPU template
is used as is w/o any changes - in the end it's a beauty and competitive
advantage of ARC cores to be tailored for a particular use-case - and
so it doesn't make a lot of sense to offer template-based "-mcpu" selection.
Given for each and every platform we end-up adding quite a few more flags
it's logical to move "-mcpu" selection to platform's definition as well
which we exactly do here.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
diff --git a/board/synopsys/emsdp/config.mk b/board/synopsys/emsdp/config.mk
index 67fd7bf..5bd1044 100644
--- a/board/synopsys/emsdp/config.mk
+++ b/board/synopsys/emsdp/config.mk
@@ -1,2 +1,2 @@
-PLATFORM_CPPFLAGS += -mlittle-endian -mnorm -mswap -mmpy-option=3 \
+PLATFORM_CPPFLAGS += -mcpu=arcem -mlittle-endian -mnorm -mswap -mmpy-option=3 \
                      -mbarrel-shifter -mfpu=fpuda_all -mcode-density