Handle -Wtruncation for link-time jr values

Fixes #2028

test/link/jr-truncation/a.asm

@@ -0,0 +1,2 @@
+SECTION "jr", ROM0
+	jr CONSTANT

test/link/jr-truncation/b.asm

@@ -0,0 +1 @@
+EXPORT DEF CONSTANT EQU $1234_0078

test/link/jr-truncation/out.err

@@ -0,0 +1,2 @@
+warning: Value $12340078 is not 16-bit [-Wtruncation]
+    at jr-truncation/a.asm(2)

test/link/jr-truncation/ref.out.sym

@@ -0,0 +1,2 @@
+; File generated by rgblink
+12340078 CONSTANT

test/link/test.sh

@@ -201,6 +201,17 @@ tryCmpRom "$test"/ref.out.bin
 tryDiff "$test"/ref.out.sym "$outtemp2"
 evaluateTest
 
+test="jr-truncation"
+startTest
+"$RGBASM" -o "$otemp" "$test"/a.asm
+"$RGBASM" -o "$outtemp" "$test"/b.asm
+continueTest
+rgblinkQuiet -o "$gbtemp" -n "$outtemp2" "$otemp" "$outtemp" 2>"$outtemp3"
+tryDiff "$test"/out.err "$outtemp3"
+tryCmpRom "$test"/ref.out.bin
+tryDiff "$test"/ref.out.sym "$outtemp2"
+evaluateTest
+
 test="high-low"
 startTest
 "$RGBASM" -o "$otemp" "$test"/a.asm