kbuild: fix modpost warnings for xtensa

The Xtensa architecture places literal pools in sections separate
from the instructions. The corresponsing text sections, therefore,
reference the .literal section, and we have to suppress those
warnings.

The naming convention defines the name for a literal
section as .SECTION.literal, unless .SECTION is .text. In that case
the name is only .literal. Using strncmp() instead of strcmp()
to compare the from-section with .SECTION.init.refok in pattern 0
should not cause any regressions for other architectures.

We also need to suppress warnings for two informational
sections (.xt.lit and .xt.prop) used by the Xtensa architecture.

Signed-off-by: Chris Zankel <chris@zankel.net>
Signed-off-by: Sam Ravnborg <sam@ravnborg.org>