build: Fix Solaris application of linker flag -lrt in bsl.

Previously, `-lrt` was added to `BDE_ENDLDFLAGS` in `bsl.defs` without
being enclosed by `$(BDE_DYNAMIC)` and `$(BDE_STATIC)` flags.  This
causes the linker to fail because a static `librt` does not exist.

```
[bsls_alignedbuffer.t.cpp.79.o (ERROR)] <<<<<<<<<<
ld: fatal: library -lrt: not found
ld: fatal: file processing errors. No output written to bde/build/groups/bsl/bsls/bsls_alignedbuffer.t
>>>>>>>>>>
```

Unneeded flags in `bsl.defs` for DG-UX and the old Solaris Forte6
compiler are removed.

Tested that full build of all code and test drivers works on Solaris with these changes.