Add save/restore_irq

Factored irq functions into a separate file.
stmhal: Make enable_irq and disable_irq inline functions.

These functions are generally 1 machine instruction, and are used in
critical code, so makes sense to have them inline.

Also leave these functions uninverted (ie 0 means enable, 1 means
disable) and provide macro constants if you really need to distinguish
the states.  This makes for smaller code as well (combined with
inlining).

Applied to teensy port as well.