mpd: fix build for ARC

mpd: fix build for ARC

mpd package fails for both internal and external ARC toolchain as check
for pthread support fails. Such checks fails because _REENTRANT flag is
not defined in gcc even when -pthread is passed.

So we add patch to gcc that defines _REENTRANT  on ARC when -pthread is
passed.

Also it disables mpd package for external ARC toolchain as it fails due
to the same issue.

This patch should be reverted as soon as the patch for GCC becomes a
part of ARC toolchain.

Fixes:
  http://autobuild.buildroot.net/results/7d7/7d70b62ad996830fbeca46dffcc7a1dc030e575d//

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