sanitize rt_sigaction() situation a bit

Switch from __ARCH_WANT_SYS_RT_SIGACTION to opposite
(!CONFIG_ODD_RT_SIGACTION); the only two architectures that
need it are alpha and sparc.  The reason for use of CONFIG_...
instead of __ARCH_... is that it's needed only kernel-side
and doing it that way avoids a mess with include order on many
architectures.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

arch/ia64/include/asm/unistd.h

@@ -49,10 +49,6 @@ asmlinkage unsigned long sys_mmap2(
 struct pt_regs;
 struct sigaction;
 asmlinkage long sys_ia64_pipe(void);
-asmlinkage long sys_rt_sigaction(int sig,
-				 const struct sigaction __user *act,
-				 struct sigaction __user *oact,
-				 size_t sigsetsize);
 
 /*
  * "Conditional" syscalls