Add link-time RST instruction

This allows using a label as the argument to a `rst` instruction
Fixes rednex#448

test/asm/rst.asm

@@ -0,0 +1,35 @@
+
+SECTION "calls", ROM0[0]
+
+; The values are not known at this point, forcing the assembler to emit an
+; expression
+	rst rst00
+	rst rst08
+	rst rst10
+	rst rst18
+	rst rst20
+	rst rst28
+	rst rst30
+	rst rst38
+
+	rst rst2A
+
+
+defRST: MACRO
+; FIXME: This is required, otherwise the lexer does not paste the two tokens
+ADDR equs "$\1"
+SECTION "rst\1", ROM0[ADDR]
+
+rst\1:
+	PURGE ADDR
+ENDM
+	defRST 00
+	defRST 08
+	defRST 10
+	defRST 18
+	defRST 20
+	defRST 28
+	defRST 30
+	defRST 38
+
+	defRST 2A ; Define a nonsensical RST, because RGBASM cannot catch it