Add CycleTimer implementation for M68K architecture (#1050)

As per discussions in here [1], LLVM is going to get backend support on
Motorola 68000 series CPUs (a.k.a M68K or M680x0). So it's necessary to
add CycleTimer implementation here, which is simply using `gettimeofday`
same as MIPS. This fixes #1049

[1] https://reviews.llvm.org/D88389

AUTHORS

diff --git a/AUTHORS b/AUTHORS
index e353b53..3068b2e 100644
--- a/AUTHORS
+++ b/AUTHORS
@@ -55,3 +55,4 @@
 Yixuan Qiu <yixuanq@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
 Zbigniew Skowron <zbychs@gmail.com>
+Min-Yih Hsu <yihshyng223@gmail.com>

CONTRIBUTORS

diff --git a/CONTRIBUTORS b/CONTRIBUTORS
index 6beed71..7071d7b 100644
--- a/CONTRIBUTORS
+++ b/CONTRIBUTORS
@@ -77,3 +77,4 @@
 Yixuan Qiu <yixuanq@gmail.com>
 Yusuke Suzuki <utatane.tea@gmail.com>
 Zbigniew Skowron <zbychs@gmail.com>
+Min-Yih Hsu <yihshyng223@gmail.com>

src/cycleclock.h

diff --git a/src/cycleclock.h b/src/cycleclock.h
index 179c67c..93d579a 100644
--- a/src/cycleclock.h
+++ b/src/cycleclock.h
@@ -161,7 +161,7 @@
   struct timeval tv;
   gettimeofday(&tv, nullptr);
   return static_cast<int64_t>(tv.tv_sec) * 1000000 + tv.tv_usec;
-#elif defined(__mips__)
+#elif defined(__mips__) || defined(__m68k__)
   // mips apparently only allows rdtsc for superusers, so we fall
   // back to gettimeofday.  It's possible clock_gettime would be better.
   struct timeval tv;