arm: work around assembler bug

Disable sibling call optimization based on binutils version.  This is
to work around a bug in the assember in binutils versions < 2.22.
Branches to weak symbols can be incorrectly optimized in thumb mode to
a short branch (b.n instruction) that won't reach when the symbol gets
preempted.

http://sourceware.org/bugzilla/show_bug.cgi?id=12532



Signed-off-by: Allen Martin <amartin@nvidia.com>
Acked-by: Tom Rini <trini@ti.com>