[git commit ldso-future] cacheflush.c: compile only if __NR_cacheflush is available

[Peter S. Mazinger]

commit: http://git.uclibc.org/uClibc/commit/?id=f77c1955ed9ca26c1474e19aeb67e8548466c069
branch: http://git.uclibc.org/uClibc/commit/?id=refs/heads/ldso-future

The header had to be removed under the guard, else asm/cachectl.h might
not be available at all.
While there, disable __cachectl prototype.

Signed-off-by: Peter S. Mazinger <ps.m at gmx.net>

Patch is too large, so refusing to show it