Bug 1325771 - mfbt:tests: Define RETURN_INSTR for m68k in TestPoisonArea r=glandium

Define RETURN_INSTR for m68k in TestPoisonArea, i.e. the m68k assembly
opcodes for "rts ; rts".

Depends on D77289

Differential Revision: https://phabricator.services.mozilla.com/D77290

mfbt/tests/TestPoisonArea.cpp

@@ -132,6 +132,9 @@
 #elif defined _ARCH_PPC || defined _ARCH_PWR || defined _ARCH_PWR2
 #  define RETURN_INSTR 0x4E800020 /* blr */
 
+#elif defined __m68k__
+#  define RETURN_INSTR 0x4E754E75 /* rts; rts */
+
 #elif defined __sparc || defined __sparcv9
 #  define RETURN_INSTR 0x81c3e008 /* retl */