xtensa: shut up gcc-8 warnings

xtensa: shut up gcc-8 warnings

Many uses of strncpy() on xtensa causes  a warning like

arch/xtensa/include/asm/string.h:56:42: warning: array subscript is above array bounds [-Warray-bounds]
   : "0" (__dest), "1" (__src), "r" (__src+__n)

This avoids the warning by turning the pointer arithmetic into an
integer operation that does not get checked the same way.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>