xtensa: Add -mforce-no-pic option is supported

GCC is configured to always compile with PIC enabled for the Xtensa
architecture. This fails when nfsroot is enabled as the code uses a
non-conformant segment that mismatches in permissione with a read-only
segment.
A patch has been submitted to GCC to add the machine-option 'force-no-pic'
that disables PIC.

Signed-off-by: Chris Zankel <chris@zankel.net>