glibc: update glibc to 2.26+ and switch to download from git

glibc: update glibc to 2.26+ and switch to download from git

Switch glibc to 2.26+ [0] and download the sources from git which will it
make easier to update glibc in the future.

For an overview of the fixed bugs and CVE related fixes in 2.26+ see the
NEWS file [1]

[0] https://sourceware.org/git/?p=glibc.git;a=commit;h=d300041c533a3d837c9f37a099bcc95466860e98
[1] https://sourceware.org/git/?p=glibc.git;a=blob;f=NEWS;h=410f0d163145c1549fb73fdd96568f585b5bf62f;hb=refs/heads/release/2.26/master

Signed-off-by: Xinxing Hu <xinxing.huchn@gmail.com>
Signed-off-by: Hans Dedecker <dedeckeh@gmail.com>