toolchain: glibc: bump to version 2.27

toolchain: glibc: bump to version 2.27

Support for RISC-V ISA was added in this release.  Other changes can be
reviewed in NEWS file of the source code [1].

The other thing is that we now switch back to preparing build dir by
using release tarballs plus patches.  The glibc repo has a long history
and is quite big in size (166MB as of 2018-04-28) and git proto lacks a
"resume clone" feature, these combined together actually makes each
minor version bump a frustrating experience for users with small
bandwidth and feeble internet connection.

The patches were generated with the following commands in a local glibc
repo and the procedure should be mostly reproducible

    git format-patch \
	--output-directory ~/git-repo/lede-project/lede/toolchain/glibc/patches \
	--no-signature \
	glibc-2.27..origin/release/2.27/master \
      | while read ff; do
	    d=$(dirname $ff)
	    f=$(basename $ff)
	    i=${f%%-*}
	    j=${f#*-}
	    mv $ff "$d/$i-2.27-$j"
	done

[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=NEWS;hb=refs/heads/release/2.27/master

Signed-off-by: Yousong Zhou <[email protected]>