META MODE: Don't double stage SYMLINKS for shared libraries.

This also fixes truncating the path that the links were installed to, which
was most likely going to be the same directory the library was in anyhow.

Let bsd.sys.mk handle SYMLINKS via stage_symlinks. stage_libs continues to
handle the SHLIB_LINK though since it is not a SYMLINKS.

This fixes a race, seen easily in lib/libthr, where libpthread_p.a is created
by both stage_libs and stage_symlinks resulting in 'ln: File exists'.

Sponsored by: EMC / Isilon Storage Division
Discussed with: sjg