xtensa: always emit .literal_position for call0 ABI

xtensa assembler implicitly starts literal pool before entry
instructions. Emit .literal_position at the beginning of function for
call0 ABI explicitly, as otherwise direct calls to external symbols
relaxed by assembler will break build when text section literals are
enabled.

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