As reported in #14430 the Xtensa compiler can add R_XTENSA_ASM_EXPAND
relocation relaxation entries in object files, and they were not
supported by mpy_ld.
This commit adds handling for that entry, doing nothing with it, as it
is only of real use for an optimising linker.
Signed-off-by: Alessandro Gatti <a.gatti@frob.it>