install(TARGETS): Do not apply installation tweaks to NAMELINK files

These files are symlinks to the real binaries, and we already apply
tweaks to those.  Previously we generated installation tweak code
guarded by a `NOT IS_SYMLINK` condition that is never true.  Drop the
code altogether.

Add a test covering the motivating use case, in which a `POST_BUILD`
step modifies the namelink file to not actually be a symlink.

Fixes: #24647