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.

Unneeded flags in `bsl.defs` for DG-UX and the old Solaris Forte6
compiler are removed.
[APPROVAL] sunos-cxx-rt-fixes-drqs-59054153 to master