Add more test coverage for RGBASM (#1256)

This also fixes two bugs: `-1 >>> 32` was -1 not 0, and `macro_FreeArgs` should have been called but wasn't.

test/asm/purge-multiple.asm

@@ -0,0 +1,9 @@
+SECTION "s", ROM0
+u::
+def v = 0
+def w equ 1
+def x equs "2"
+MACRO y
+ENDM
+; purge many symbols at once to test parser reallocation
+PURGE u, v, w, x, y