consolidate cond_syscall and SYSCALL_ALIAS declarations

take them to asm/linkage.h, with default in linux/linkage.h

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

arch/mips/include/asm/unistd.h

@@ -63,12 +63,4 @@
 
 #endif /* !__ASSEMBLY__ */
 
-/*
- * "Conditional" syscalls
- *
- * What we want is __attribute__((weak,alias("sys_ni_syscall"))),
- * but it doesn't work on all toolchains, so we just do it by hand
- */
-#define cond_syscall(x) asm(".weak\t" #x "\n" #x "\t=\tsys_ni_syscall")
-
 #endif /* _ASM_UNISTD_H */