HEADS UP: Clang updated again, fixes module issues

This weekend I updated our copy of llvm/clang again, from trunk r126079
to trunk r126547.

There are several bugfixes in this update, but the most important one is
to ensure __start_ and __stop_ symbols for linker sets and kernel module
metadata are always emitted in object files.

Before this fix, if you compiled kernel modules with clang, they would
not be properly processed by kldxref, and if they had any dependencies,
the kernel would fail to load those.  Another problem occurred when
attempting to mount a tmpfs filesystem, which would result in 'operation
not supported by device'.

So, if you compiled your system with clang r126079 (the one from last
week), please rebuild it again, if you want to have properly loading
modules.