as7: accept (decimal) digits in octal constants, like as.s does!

[philbudne]

Alternative to changing "bad" octal constants in source files, such as one in st7.s: https://github.com/DoctorWkt/pdp7-unix/pull/224

[sebras]

I tried to compile this file using as from the image:

o20: 020
o21: 021
o22: 022
o23: 023
o24: 024
o25: 025
o26: 026
o27: 027
o28: 028
o29: 029
o30: 030
0;0;0;0;0

no20: -020
no21: -021
no22: -022
no23: -023
no24: -024
no25: -025
no26: -026
no27: -027
no28: -028
no29: -029
no30: -030

0;0;0;0;0

as expected it the a.out looks like this:

@ od a.out
000000: 000020 000021 000022 000023 000024 000025 000026 000027 
000010: 000030 000031 000030 000000 000000 000000 000000 000000 
000020: 777760 777757 777756 777755 777754 777753 777752 777751 
000030: 777750 777747 777750 000000 000000 000000 000000 000000 
@ 

When experimenting I came up with this change that makes as7 behave similar to as. Moreover my change looks show some similarities to the one you propose

diff --git a/tools/as7 b/tools/as7
--- a/tools/as7
+++ b/tools/as7
@@ -445,6 +445,15 @@ sub parse_line {
     } # while
 }

+sub as_oct {
+    my $oct = shift;
+    my $value = 0;
+    for my $c (split //, $oct) {
+           $value = $value * 8 + $c;
+    }
+    return $value;
+}
+
 # Blame Phil for this bit too...
 # Parse an expression off $line and return a PDP-7 word
 # as a series of whitespace separated "syllables"
@@ -514,7 +523,7 @@ sub parse_expression {
            my $value = $1;
            printf "\tfound constant: $value\n" if ($debug);
            if ( $value =~ m{^0} ) {
-               $syllable = oct($value);
+               $syllable = as_oct($value);
            }
            else {
                $syllable = $value + 0;

Compiling my test code above using as7 with your commit appears to produce the same output as as on target so it looks safe to merge.