kbuild: fix asm-offset generation to work with clang
KBuild abuses the asm statement to write to a file and
clang chokes about these invalid asm statements. Hack it
even more by fooling this is actual valid asm code.
[masahiro:
Import Jeroen's work for U-Boot:
http://patchwork.ozlabs.org/patch/375026/
Tweak sed script a little to avoid garbage '#' for GCC case, like
#define NR_PAGEFLAGS 23 /* __NR_PAGEFLAGS # */ ]