Update libuv to rebase on top of joyent/master

Closes #8829
Enable segmented stacks for llvm on mingw-64

cc #8488
auto merge of #8840 : alexcrichton/rust/update-vendors, r=cmr

The LLVM update includes patches from #8488 by @klutzy to build llvm on mingw-64 and also to enable segmented stacks on that platform.

The libuv patch is a rebase on the now-current joyent/master in order to fix #8829