xtensa: move coprocessor_flush to the .text section

coprocessor_flush is not a part of fast exception handlers, but it uses
parts of fast coprocessor handling code that's why it's in the same
source file. It uses call0 opcode to invoke those parts so there are no
limitations on their relative location, but the rest of the code calls
coprocessor_flush with call8 and that doesn't work when vectors are
placed in a different gigabyte-aligned area than the rest of the kernel.

Move coprocessor_flush from the .exception.text section to the .text so
that it's reachable from the rest of the kernel with call8.

Cc: stable@vger.kernel.org
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>