Reset LOAD offset when changing SECTIONs

This would cause spurious section overflow messages, since the load offset
is added to the section's when computing its size.

test/asm/load-pushs.asm

@@ -0,0 +1,15 @@
+
+SECTION "Output", ROM0[0]
+    db SIZEOF("ROM CODE")
+    db SIZEOF("RAM CODE")
+    db SIZEOF("HRAM")
+
+
+SECTION "ROM CODE",ROM0
+    ds $80
+LOAD "RAM CODE",SRAM
+PUSHS
+SECTION "HRAM",HRAM
+    ds 1
+POPS
+ENDL