valgrind: fix mips64 compile

Disable gcc march mips64r2 detection (use compile flags already
set by buildroot only), avoids double setting like '-march=mips64
... -march=mips64r2 -mabi=64'.

Fixes [1]:

  error: '-mips64r2' conflicts with the other architecture options, which specify a mips64 processor

[1] http://autobuild.buildroot.net/results/34f6e2352f1559f98c724fe5394db0035b42ddb1



Signed-off-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>