Make SIMD capability variables thread-local ...

... on platforms that support TLS, which should include all
currently-supported platforms
(https://libjpeg-turbo.org/Documentation/OfficialBinaries)

Addresses a concern raised in #87

Although it is still my opinion that the data race in init_simd() was
innocuous, we can now fix it for free thanks to
ae87a95861, so why not?

simd/mips64/jsimd.c

@@ -2,7 +2,7 @@
  * jsimd_mips64.c
  *
  * Copyright 2009 Pierre Ossman <ossman@cendio.se> for Cendio AB
- * Copyright (C) 2009-2011, 2014, 2016, 2018, D. R. Commander.
+ * Copyright (C) 2009-2011, 2014, 2016, 2018, 2022, D. R. Commander.
  * Copyright (C) 2013-2014, MIPS Technologies, Inc., California.
  * Copyright (C) 2015, 2018, Matthieu Darbois.
  * Copyright (C) 2016-2018, Loongson Technology Corporation Limited, BeiJing.
@@ -26,7 +26,7 @@
 
 #include <ctype.h>
 
-static unsigned int simd_support = ~0;
+static THREAD_LOCAL unsigned int simd_support = ~0;
 
 #if defined(__linux__)
 
@@ -94,8 +94,6 @@ parse_proc_cpuinfo(int bufsize)
 
 /*
  * Check what SIMD accelerations are supported.
- *
- * FIXME: This code is racy under a multi-threaded environment.
  */
 LOCAL(void)
 init_simd(void)