Handle some .OBJDIR == .CURDIR cases.

Handle some .OBJDIR == .CURDIR cases.

- If OBJROOT is SRCTOP then don't add on TARGET.TARGET_ARCH.  This
  only happens at the top-level, and for sub-directories when the
  user is clever with MAKEOBJDIRPREFIX=/.
- Don't bother checking 'test -w' on .CURDIR.
- Properly set OBJTOP/OBJROOT to SRCTOP in various needed cases.
- Check if the OBJDIR is writable even for *clean* targets since it
  determines which .OBJDIR the user gets;  If they cannot write to an
  existing eligible .OBJDIR then it needs to clean in .CURDIR instead.
- Add guard to cleanworld/cleanuniverse from removing SRCTOP.
- Ensure OBJTOP is proper for .OBJDIR=.CURDIR which fixes finding
  libraries since src.libnames.mk is based on OBJTOP.
- Avoid some chdir(2) for modifying .OBJDIR

Sponsored by:	Dell EMC Isilon