define and use internal macros for hidden visibility, weak refs

this cleans up what had become widespread direct inline use of "GNU C"
style attributes directly in the source, and lowers the barrier to
increased use of hidden visibility, which will be useful to recovering
some of the efficiency lost when the protected visibility hack was
dropped in commit dc2f368e56, especially
on archs where the PLT ABI is costly.

arch/mips/syscall_arch.h

@@ -3,8 +3,7 @@
 ((union { long long ll; long l[2]; }){ .ll = x }).l[1]
 #define __SYSCALL_LL_O(x) 0, __SYSCALL_LL_E((x))
 
-__attribute__((visibility("hidden")))
-long (__syscall)(long, ...);
+hidden long (__syscall)(long, ...);
 
 #define SYSCALL_RLIM_INFINITY (-1UL/2)