Update ifunc attribute support documentation

[emaste]

Summary

We documented GNU binutils and glibc versions required for ifunc support, but our own lld linker and FreeBSD's rtld also support ifuncs.

[DmitryPolukhin]

LGTM

[joerg]

I think this is still too optimistic. Full support for ifunc seems to be generally limited to x86. Most other architectures lack even definitions for anonymous ifunc relocations or support proper relaxation only in limited forms. That's especially annoying when looking at static linking.

[emaste]

Maybe "available for some architectures in at least..."? Or maybe we shouldn't bother trying to list versions, and mention it is dependent on CPU arch, linker, and rtld?

[joerg]

Yeah, I would restrict it to just mention that it depends on the target, link time editor and runtime linker. Even the concrete feature set on Linux changes with glibc versions.

include/clang/Basic/AttrDocs.td

def IFuncDocs : Documentation {
   let Category = DocCatFunction;
   let Content = [{
 ``__attribute__((ifunc("resolver")))`` is used to mark that the address of a declaration should be resolved at runtime by calling a resolver function.
 
 The symbol name of the resolver function is given in quotes.  A function with this name (after mangling) must be defined in the current translation unit; it may be ``static``.  The resolver function should return a pointer.
 
 The ``ifunc`` attribute may only be used on a function declaration.  A function declaration with an ``ifunc`` attribute is considered to be a definition of the declared entity.  The entity must not have weak linkage; for example, in C++, it cannot be applied to a declaration if a definition at that location would be considered inline.
 
-Not all targets support this attribute.  ELF targets support this attribute when using binutils v2.20.1 or higher and glibc v2.11.1 or higher.  Non-ELF targets currently do not support this attribute.
+Not all targets support this attribute.  ELF target support depends on both the linker and runtime linker, and is available in at least lld 4.0 and later, binutils 2.20.1 and later, glibc v2.11.1 and later, and FreeBSD 9.1 and later.  Non-ELF targets currently do not support this attribute.
   }];
 }
 
 def LTOVisibilityDocs : Documentation {
   let Category = DocCatType;
   let Content = [{
 See :doc:`LTOVisibility`.
   }];