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.