Always try to set .OBJDIR, even if AUTO_OBJ is enabled, and fallback to .CURDIR.

When AUTO_OBJ is enabled this change becomes redundant with the auto.obj.mk
check added in r325404. However, it is possible that new code is added
at some point between src.sys.obj.mk and auto.obj.mk that disables AUTO_OBJ.
That could leave make with a bogus and unsafe .OBJDIR in some cases.

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