libthr: fix warnings from GCC when WARNS=6

[vangyzen]

Summary

Fix warnings about redundant declarations, a local variable shadowing
a global function (dlinfo), and an old-style function definition
(with an empty parameter list).

Test Plan

make tinderbox passes (except for a few unrelated kernel failures)

[vangyzen]

I added _.riscv.* to the svn:ignore property. I'll commit that separately.

[kib]

lib/libthr/thread/thr_private.h

873

Since the #if's become nested, I would consider adding comments to #endif lines.

[vangyzen]

include/stdio.h

467

I don't particularly like this, but it follows precedent.

An alternative would move the declaration into a common header file, but I couldn't identify a good candidate, and I didn't think creating one was worth the effort.

I'm open to suggestion, of course.

lib/libthr/thread/thr_private.h

841

I don't particularly like this. It requires consumers to include libc_private.h before thr_private.h. It also adds maintenance, because additions to libc_private.h might require adjustments here. However, given the limited usage of these files, both of these requirements seem fairly reasonable.

An alternative would create a new, common header file, such as libc_thr_private.h, included by libc_private.h and thr_private.h.

I'm open to suggestion, of course.