package/glibc: switch to using the maintenance branch

glibc upstream has ruled against doing regular point-releases, but they
do have a lot of interesting and important fixes for regressions and
security.

Backporting each patch, or cherry-picking individual patches is off
limits for us, so we just switch to using the currently-latest HEAD of
the maintenance branch instead.

The version number is obtained with:
    $ git describe --match 'glibc-*' --abbrev=40 origin/release/2.26/master

The alternative options were:
  - download the tarball from the git tree
    --> does not work; not an option
  - download the 2.26 tarball, and bundle the individual patches in
    Buildroot
    --> maintenance of patches is a burden; not an option
  - download the 2.26 tarball, maintain the list of patches to download from
    the git tree
    --> not an option for the same reason

So we end up just doing a git clone. The git tree is today about ten
times the size of the tarball, so a rough estimate makes it at about ten
times the download time.

Also upstream doesn't officially provide an https download location [1].
There is one but it's not reliable, sometimes the connection time out and
end-up with a corrupted git repo:

fatal: unable to access 'https://sourceware.org/git/glibc.git/': Failed to connect to sourceware.org port 443: Connection timed out

So switch to using a git mirror from github which is updated once a day [2].
This allow at the same time to clone the git repository faster.

Note: The glibc 2.26 patches are not kept for the arc toolchain since they
are fixing an issue with the new float128 support introduced in x86, x86_64
and powerpc64le.

[1] https://sourceware.org/git/?p=glibc.git;a=summary
[2] https://github.com/bminor/glibc.git

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Cc: Romain Naour <romain.naour@openwide.fr>
Cc: Peter Korsgaard <peter@korsgaard.com>
Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Arnout Vandecappelle <arnout@mind.be>
Cc: Evgeniy Didin <didin@synopsys.com>
CC: Alexey Brodkin <abrodkin@synopsys.com>
[Romain: bump 4b692dffb95ac4812b161eb6a16113d7e824982e]
Signed-off-by: Romain Naour <romain.naour@gmail.com>
[yann.morin.1998@free.fr: update comment to never decide on the mirror]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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