xtensa: implement jump_label support

Use 3-byte 'nop' and 'j' instructions that are always present. Don't let
assembler mark a spot right after patchable 'j' instruction as
unreachable and later put literals or padding bytes there. Add separate
implementations of patch_text for SMP and UP cases, avoiding use of
atomics on UP.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>