[Bug lto/48200] Implement function attribute for symbol versioning (.symver)

[xry111 at mengyan1223 dot wang]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=48200

--- Comment #30 from Xi Ruoyao <xry111 at mengyan1223 dot wang> ---
Pick up this again...

Now __asm__(".symver a, b@@ver") is used for two different purposes:

1. Mark a function/object *definition* as the specified version.
2. "Import" a symbol (function/object) with the specified version.

If we make up two attributes for those two purposes, life will be easier.  For
1 we can emit ".symver" directive when the definition is emited.  For 2 we can
borrow the logic from .weakref and do minimal change.