xtensa: don't use l32r opcode directly

xtensa assembler is capable of representing register loads with either
movi + addmi, l32r or const16, depending on the core configuration.
Don't use '.literal' and 'l32r' directly in the code, use 'movi' and let
the assembler relax them.

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