The ebuild for dev-util/valgrind-3.4.1-r1 uses "make install", rather than "emake install", so the resulting make is not parallel, even when MAKEOPTS specifies parallel operation. However, the install target is parallel safe. After switching to "emake install", I can successfully execute src_install with MAKEOPTS=-j. I deliberately push MAKEOPTS to unlimited mode for the src_install phase to try to provoke failures. This is rated trivial since parallel installation is less important than parallel compilation, but it would still be nice to get it fixed. All that is required is to change src_install to use emake, rather than make.
Fixed. Thanks for the report.