arc: make global_data.h usable in assembly files

Currently on attempt to use global_data.h in an assembly file following
will happen:
-------------------->8-----------------
./arch/arc/include/asm/global_data.h: Assembler messages:
./arch/arc/include/asm/global_data.h:11: Error: bad instruction 'struct arch_global_data{'
./arch/arc/include/asm/global_data.h:12: Error: junk at end of line, first unrecognized character is `}'
scripts/Makefile.build:316: recipe for target 'arch/arc/lib/start.o' failed
-------------------->8-----------------

In this change we disable struct arch_global_data in ASM which fixes
the issue above.

Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>