Fix some fixed-point literal lexing issues (#1914)

- No fractional digits are necessary after the decimal point,
  e.g. `42.` is valid instead of `42.0`
- Error messages refer to "fixed-point" not "integer" constants
- Test more carefully for lexing unrelated to underscores

test/asm/fixed-point-syntax.out

@@ -0,0 +1,10 @@
+$323D7
+$2A0000
+$4D2915B
+$18B
+$62B85
+$4
+$C570A
+$13333
+$13333
+$13333