Convert all assembly including inline to `.syntax unified`

[kilograham]

The majority of our .S files use unified syntax, however very little inline assembler does.

This can lead to bugs, e.g. this

__force_inline static int32_t __mul_instruction(int32_t a, int32_t b) {
    asm ("mul %0, %1" : "+l" (a) : "l" (b) : );
    return a;
}

which is actually a muls instruction, and sets the flags, so the condition codes need to be added to the clobbers.

We can't/shouldn't change the default inline assembly syntax to unified since this is really the purview of the user. instead we should just explicitly set it on all inline assembly.