valgrind: bump to 3.8.1

Status of the patches:

 * valgrind-compiler-check.patch, no longer needed, merged upstream.

 * valgrind-dont-include-a-out-header.patch, kept, just refreshed

 * valgrind-fix-ccache-support.patch, adapted to the newer Valgrind
   release.

 * valgrind-largefile.patch, kept, just refreshed

 * valgrind-more-ioctls.patch, removed. Most of it was merged
   upstream. This patch was anyway a feature addition, so it shouldn't
   be kept in Buildroot.

 * valgrind-workaround-SIGSEGV-on-PPC.patch, kept as is, just
   refreshed.

This bump also fixes the build failure we were experiencing with
Valgrind 3.7.0 against recent Glibc versions.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>