Use pkglibexec as vglibdir.

Use pkglibexec as vglibdir.

vglibdir is the directory from where valgrind loads its internal tool
executables and vgpreloads. Currently vglibdir is pkglibdir, so those
internal tools are intermingeled with normal executables and libraries
that the user might use directly.

Make vglibdir equal to pkglibexecdir so the internal tools get installed
and loaded from libexec and don't get get stored under lib.

This leaves just the static archives and the mpiwrapper libraries that
the user would link/load themselves under pkglibdir.

This seems more in line with the FHS lib/libexec standard and makes it
slightly easier to combine the tools from a multilib target (say the
memcheck-amd64-linux and memcheck-x86-linux tools) because they would
be installed under the same directory, while the pkglibdir can differ
depending on arch/target (lib/lib64).

https://bugs.kde.org/show_bug.cgi?id=433323