This patch avoids assembler warnings for gfx908 and gfx90a such as
   '-xnack-mattr=-sramecc' is not a recognized feature for this target(ignoring feature)
as we pass   -mattr=-xnack-mattr=-sramecc  to the llvm-mc assembler.

Solution: Add a space before the second '-mattr='.

OK for mainline?

Tobias