Remove unneeded _syscallX macros from user view in asm-*/unistd.h

These aren't needed by glibc or klibc, and they're broken in some cases
anyway. The uClibc folks are apparently switching over to stop using
them too (now that we agreed that they should be dropped, at least).

Signed-off-by: David Woodhouse <dwmw2@infradead.org>