musl: update musl to 1.1.16+ and switch to download from git

This patch updates musl to 1.1.16+ [0] and removes all the
backported patches. This is a major release and tagged as such.
For more information visit musl-libc.org or read the WHATSNEW.

Furthermore, this patch also changes musl to download directly
from git. This makes it easier to update musl in the future.

The patch custom Add-format-attribute-to-some-function-declarations.patch
was assigned a new 400- number. This should avoid confusion
since 0xx numbers are usually assigned to backports.

[0] <http://git.musl-libc.org/cgit/musl/commit/?id=769f53598e781ffc89191520f3f8a93cb58db91f

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Cc: Hannu Nyman <hannu.nyman@iki.fi>
Cc: Koen Vandeputte <koen.vandeputte@ncentric.com>
Cc: Jo-Philipp Wich <jo@mein.io>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Signed-off-by: Christian Lamparter <chunkeey@googlemail.com>