Only emit inline-assembly when building for AVR

Make sure that we'll never emit AVR assembly on non-AVR targets.
Instead of failing the build, fail at runtime, to allow a potential
application testsuite to run even if those functions somehow get linked
in.

Signed-off-by: Rahix <rahix@rahix.de>

Add asm module for assembly instructions

Add a new module containing wrapper functions for various assembly
instructions.  These functions are marked inline(always) to ensure they
will always just generate the single instruction as expected.

Signed-off-by: Rahix <rahix@rahix.de>