xtensa: fix build for cores with coprocessors

Assembly entry/return abstraction change didn't add asmmacro.h include
statement to coprocessor.S, resulting in references to undefined macros
abi_entry and abi_ret on cores that define XTENSA_HAVE_COPROCESSORS.
Fix that by including asm/asmmacro.h from the coprocessor.S.

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