package/valgrind: bump to version 3.15.0

package/valgrind: bump to version 3.15.0

Additionally, backport an upstream patch with which valgrind can now
be compiled for MIPS32r6/MIPS64r6 and reenable valgrind for those
architectures in the Config.in file.

Moreover, remove a patch which is not needed anymore since
https://bugs.kde.org/show_bug.cgi?id=400975 has been resolved, also
making AUTORECONF = YES no longer necessary.

Signed-off-by: Stefan Maksimovic <stefan.maksimovic@rt-rk.com>
[Thomas: improve commit log, add reference to upstream commit]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>