gstreamer: fix build on riscv

RISC-V supports unaligned accesses, but these might run extremely slowly
depending on the implementation so set as_cv_unaligned_access to no on
this architecture, see:
https://gitlab.freedesktop.org/gstreamer/gstreamer/commit/8a156d1725ecd03f2e8cdc8874e081dda2d3b43d

Fixes:
 - http://autobuild.buildroot.org/results/dca32c929350311fca1a85e251d318b43c4a3177



Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Reviewed-by: Peter Seiderer <ps.report@gmx.net>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>