alpha.h (TARGET_WINDOWS_NT, [...]): Just make them real constants, since they can...

alpha.h (TARGET_WINDOWS_NT, [...]): Just make them real constants, since they can't be changed.

        * alpha/alpha.h (TARGET_WINDOWS_NT, TARGET_OPEN_VMS): Just make them
        real constants, since they can't be changed.
        (TARGET_AS_CAN_SUBTRACT_LABELS): New.
        * alpha/alpha.md (builtin_setjmp_receiver): Use it.
        * alpha/osf.h (TARGET_AS_CAN_SUBTRACT_LABELS): New.
        * alpha/osf2or3.h (TARGET_AS_CAN_SUBTRACT_LABELS): New.
        * alpha/vms.h (TARGET_OPEN_VMS): New.
        * alpha/win-nt.h (TARGET_WINDOWS_NT): New.

From-SVN: r18417