libc_hidden_proto breaks things for PowerPC64

[Roland McGrath]

> Roland, what do you mean by "fix the compiler support", especially in
> regard to data definitions?

With all the intermediate versions we had in this stuff I lost track of the
fact that hidden_def is always done via asm.  I don't know why it's that
way (Jakub?).  I think (with some gcc fixes) it can be done using
__attribute__ like the strong_alias macro is done.  What think would be the
best overall solution is to make the __attribute__ approach work properly
in gcc for all platforms, and then use that (and I don't care if we just
punt the hidden_proto optimizations for broken compilers).  Then we don't
have to worry about things like dot-symbols and whatever the variants of
how symbols are defined when they are functions or data or whatnot--the
compiler already knows about all that and should deal with it for us.