Handle some .OBJDIR == .CURDIR cases.
rS325626
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Description

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