attr: backport upstream patches to fix build on musl

Upstream has already merged a number of patches that fix the build of
attr with the musl C library. This commit backports those two patches
in Buildroot, until upstream makes a new attr release.

Fixes:

  http://autobuild.buildroot.net/results/778/7785d07614ff1d76db79c86b790f13e8ab800cba/



Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>