As the list of architectures now grows to include x86_64, it's
easier and more robust to check for anything but VAX since all
the later architectures behave the same for the things being
ifdef'd.

Rather than adding a fourth hard-coded option to the list, let's
derive the built-in name from the architecture name we already
have.  Some of them come back from sys$getsyi in mixed case, so
upper case the result when defining the built-in.

N.B.  The existing behavior differs from MMS and is not changed
here.  MMS will set, for example, MMSALPHA to a value of "ALPHA"
whereas MMK sets it to "1".  Either should work in an IFDEF, so
it may not matter.

Otherwise we get:

$ MACRO/MIGRATION/NOLIST/OBJECT=BIN_DIR:PARSE_TABLES.OBJ -
	SYS$LIBRARY:ARCH_DEFS.MAR+SYS$DISK:[]PARSE_TABLES.MAR

EVAX	=	1
^
%XMAC-W-GENWARN, generated WARNING: 0 Architecture symbol set to 1, probably an error

VAX doesn't have this.

This rounds out some things that were missing, such as kitinstal
updates.

MMK now propely handles DCL command lines longer than 255 characters on VAX.
Such lines are broken up into continuation lines that are passed to the
worker subprocess. The code was there in previous versions, but the limit
was defined incorrectly, so the continuation lines were 257 characters long,
which was two characters too long for VAX.