Bug 1318905 - mfbt:tests: Define RETURN_INSTR for riscv64 in TestPoisonArea r=glandium

Define RETURN_INSTR for riscv64 in TestPoisonArea, i.e. the riscv64 assembly
opcodes for "ret ; ret".

Differential Revision: https://phabricator.services.mozilla.com/D78625

mfbt/tests/TestPoisonArea.cpp

@@ -135,6 +135,9 @@
 #elif defined __m68k__
 #  define RETURN_INSTR 0x4E754E75 /* rts; rts */
 
+#elif defined __riscv
+#  define RETURN_INSTR 0x80828082 /* ret; ret */
+
 #elif defined __sparc || defined __sparcv9
 #  define RETURN_INSTR 0x81c3e008 /* retl */