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>

arch/xtensa/Makefile

@@ -35,6 +35,8 @@ KBUILD_CFLAGS += -ffreestanding
 
 KBUILD_CFLAGS += -pipe -mlongcalls
 
+KBUILD_CFLAGS += $(call cc-option,-mforce-no-pic,)
+
 vardirs := $(patsubst %,arch/xtensa/variants/%/,$(variant-y))
 plfdirs := $(patsubst %,arch/xtensa/platforms/%/,$(platform-y))