GNUInstallDirs: Fix misinterpretation of Debian Policy on LIBEXECDIR

Revert commit 798c1c3192 (GNUInstallDirs: Comply with Debian Policy on
LIBEXECDIR, 2020-10-08, v3.19.0-rc1~11^2).

The Debian Policy builds upon FHS 3.0 and permits installing to
`/usr/libexec`. While Policy does grant an additional exception for
applications to use a single subdirectory of `/usr/lib/<triplet>`, this
is not meant to replace `/usr/libexec` as valid target.

Fixes: #22731