jump_label, x86: Emit short JMP

Now that we can patch short JMP/NOP, allow the compiler/assembler to
emit short JMP instructions.

There is no way to have the assembler emit short NOPs based on the
potential displacement, so leave those long for now.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20210506194157.967034497@infradead.org