clang: work around asm output constraint problems

clang: work around asm output constraint problems

Work around clang problems with "=rm" asm constraint.

clang seems to always chose the memory output, while it is almost
always the worst choice.

Add ASM_OUTPUT_RM so that we can replace "=rm" constraint
where it matters for clang, while not penalizing gcc.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Suggested-by: Uros Bizjak <ubizjak@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>