jump_label: make initial NOP patching the special case

Instead of defaulting to patching NOP opcodes at init time, and leaving
it to the architectures to override this if this is not needed, switch
to a model where doing nothing is the default. This is the common case
by far, as only MIPS requires NOP patching at init time. On all other
architectures, the correct encodings are emitted by the compiler and so
no initial patching is needed.

Signed-off-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220615154142.1574619-4-ardb@kernel.org

ARC: ARCv2: jump label: implement jump label patching

Implement jump label patching for ARC. Jump labels provide
an interface to generate dynamic branches using
self-modifying code.

This allows us to implement conditional branches where
changing branch direction is expensive but branch selection
is basically 'free'

This implementation uses 32-bit NOP and BRANCH instructions
which forced to be aligned by 4 to guarantee that they don't
cross L1 cache line boundary and can be update atomically.

Signed-off-by: Eugeniy Paltsev <Eugeniy.Paltsev@synopsys.com>
Signed-off-by: Vineet Gupta <vgupta@synopsys.com>