Treat PEA operand as imm const16

We were treating PEA's operand as an absolute address and attempting to
extend it to 24 bits.  This wasn't very successful, and was
counter-productive when the argument was actually a constant, which is
common.  Worse, if the operand appeared to reference an internal address
outside bank 0, we would output a 24-bit value, breaking 64tass and cc65.

We now treat PEA as having an immediate operand.  (It was already
classified as an "extended immediate", but the operand-address
computation was still happening.)

This will have little to no effect on projects that use OMF relocation
data, because PEA arguments are explicitly identified as symbols or
constants.

New tests were added to 20032-labels-and-symbols to exercise the code
generation failures.  A few other tests that used PEA were affected.
20152-local-variables was fixed by adding a symbolic reference, the
others remained as-is and the expected output was revised.

(issue #172)

Asm65/OpDef.cs

@@ -81,7 +81,7 @@ namespace Asm65 {
             Implied,            // OP               1
             PCRel,              // OP label         2 (BCC, BNE, ...)
             PCRelLong,          // OP label         3 (BRL)
-            StackAbs,           // OP addr          3 (PEA)
+            StackAbs,           // OP addr          3 (PEA), treat as imm const16
             StackDPInd,         // OP (dp)          2 (PEI)
             StackInt,           // OP               2 (BRK, COP)
             StackPCRelLong,     // OP label         3 (PER)