arm: Add missing it instructions

When building in thumb mode, the conditional instructions are supposed
to have 'it' instructions preceding them (unless the assembler is
set to automatically inject such).

This fixes building with clang/llvm 3.9, while the build passed
with clang/llvm 3.8 (unsure whether it actually injected such instructions
though).

Signed-off-by: Martin Storsjö <martin@martin.st>