xtensa: add -mlongcalls to CFLAGS for xtensa

The longcalls option allows calls across a greater range of addresses.

This option may degrade both code size and performance, but
the linker can generally optimize away the unnecessary overhead
when a call ends up within range

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>