Makefile: Disable stack-usage check for ARC

With the most recent tools for ARC (arc-2017.09) in case of
"naked" function compiler throws a warning:
---------------------------------->8-----------------------------
board/synopsys/hsdk/hsdk.c: In function 'hsdk_core_init_f':
board/synopsys/hsdk/hsdk.c:345:1: warning: stack usage computation not supported for this target
 }
 ^
---------------------------------->8-----------------------------

That happens because the compiler doesn't handle "naked" functions
as a special case where stack calculation shouldn't be done.

But for now until this is fixed in GCC to get clean buildman output
we're disabling stack-usage check for ARC.

See https://lists.denx.de/pipermail/u-boot/2018-April/324455.html


for more background.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Cc: Tom Rini <trini@konsulko.com>