Bug 1451292 - Better payload for arm64 breakpoint instruction. r=sstangl

At least some non-zero payloads confuse GDB and make it iloop on the
breakpoint instruction rather than break to the command line as it
should.  There seems to be no reason not to use a zero payload.

--HG--
extra : rebase_source : 6d6f9aa2911b86b02572f88948d48bc2238c6353
extra : amend_source : 9fed9235d481a9eadafc4a3e0075c9fef8b6050d

js/src/jit/arm64/MacroAssembler-arm64.cpp

@@ -237,8 +237,9 @@ MacroAssemblerCompat::profilerEnterFrame(RegisterOrSP framePtr, Register scratch
 void
 MacroAssemblerCompat::breakpoint()
 {
-    static int code = 0xA77;
-    Brk((code++) & 0xffff);
+    // Note, other payloads are possible, but GDB is known to misinterpret them
+    // sometimes and iloop on the breakpoint instead of stopping properly.
+    Brk(0);
 }
 
 // Either `any` is valid or `sixtyfour` is valid.  Return a 32-bit ARMRegister