Fix SECTION UNION alignment depending on piece order

test/link/section-union/compat/a.asm

@@ -0,0 +1,4 @@
+SECTION "ROM", ROM0
+LOAD UNION "U", WRAM0
+        ds 1
+ENDL

test/link/section-union/compat/b.asm

@@ -0,0 +1,17 @@
+SECTION "R1", WRAM0
+        ds $77
+
+SECTION "R2", WRAM0
+        ds $ef
+
+SECTION UNION "U", WRAM0
+        ds $52e
+
+SECTION UNION "U", WRAM0
+wStart::
+        ds $89
+        assert @ & $FF == 0, "wContent must be 8-bit aligned"
+        align 8
+wContent::
+        ds $111
+wEnd::

test/link/section-union/compat/script.link

@@ -0,0 +1,5 @@
+WRAM0
+        align 8
+        "R1"
+        "U"
+        "R2"

test/link/test.sh

@@ -390,6 +390,15 @@ rgblinkQuiet "$otemp" "$gbtemp" 2>"$outtemp"
 tryDiff "$test"/out.err "$outtemp"
 evaluateTest
 
+test="section-union/compat"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+"$RGBASM" -o "$gbtemp2" "$test"/b.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" -l "$test"/script.link "$otemp" "$gbtemp2"
+tryCmpRom "$test"/ref.out.bin
+evaluateTest
+
 test="section-union/good"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm