Eliminate duplicate copies of jpeg_nbits_table

ef9a4e05ba (libjpeg-turbo 1.4.x), which
was based on
https://bug815473.bmoattachments.org/attachment.cgi?id=692126
(https://bugzilla.mozilla.org/show_bug.cgi?id=815473), modified the C
baseline Huffman encoder so that it precomputes jpeg_nbits_table, in
order to facilitate sharing the table among multiple processes.
However, libjpeg-turbo never shared the table, and because the table was
implemented as a static array, f3a8684cd1
(libjpeg-turbo 1.5.x) and 37bae1a0e9
(libjpeg-turbo 2.0.x) each introduced a duplicate copy of the table for
(respectively) the SSE2 baseline Huffman encoder and the C progressive
Huffman encoder.

This commit does the following:
- Move the duplicated code in jchuff.c and jcphuff.c, originally
  introduced in 0cfc4c17b7 and
  37bae1a0e9, into a header
  (jpeg_nbits.h).
- Credit the co-author of 0cfc4c17b7.
  (Refer to https://sourceforge.net/p/libjpeg-turbo/patches/57).
- Modify the SSE2 baseline Huffman encoder so that the C Huffman
  encoders can share its definition of jpeg_nbits_table.
- Move the definition of jpeg_nbits_table into a C source file
  (jpeg_nbits.c) rather than a header, and define the table only if
  USE_CLZ_INTRINSIC is undefined and the SSE2 baseline Huffman encoder
  will not be built.
- Apply hidden symbol visibility to the shared definition of
  jpeg_nbits_table, if the compiler supports the necessary attribute.
  (In practice, only Visual C++ doesn't.)

Closes #114

See also:
https://bugzilla.mozilla.org/show_bug.cgi?id=1501523

jchuff.c

@@ -6,7 +6,7 @@
  * Lossless JPEG Modifications:
  * Copyright (C) 1999, Ken Murchison.
  * libjpeg-turbo Modifications:
- * Copyright (C) 2009-2011, 2014-2016, 2018-2023, D. R. Commander.
+ * Copyright (C) 2009-2011, 2014-2016, 2018-2024, D. R. Commander.
  * Copyright (C) 2015, Matthieu Darbois.
  * Copyright (C) 2018, Matthias Räncker.
  * Copyright (C) 2020, Arm Limited.
@@ -35,41 +35,7 @@
 #include "jchuff.h"             /* Declarations shared with jc*huff.c */
 #endif
 #include <limits.h>
-
-/*
- * NOTE: If USE_CLZ_INTRINSIC is defined, then clz/bsr instructions will be
- * used for bit counting rather than the lookup table.  This will reduce the
- * memory footprint by 64k, which is important for some mobile applications
- * that create many isolated instances of libjpeg-turbo (web browsers, for
- * instance.)  This may improve performance on some mobile platforms as well.
- * This feature is enabled by default only on Arm processors, because some x86
- * chips have a slow implementation of bsr, and the use of clz/bsr cannot be
- * shown to have a significant performance impact even on the x86 chips that
- * have a fast implementation of it.  When building for Armv6, you can
- * explicitly disable the use of clz/bsr by adding -mthumb to the compiler
- * flags (this defines __thumb__).
- */
-
-/* NOTE: Both GCC and Clang define __GNUC__ */
-#if (defined(__GNUC__) && (defined(__arm__) || defined(__aarch64__))) || \
-    defined(_M_ARM) || defined(_M_ARM64)
-#if !defined(__thumb__) || defined(__thumb2__)
-#define USE_CLZ_INTRINSIC
-#endif
-#endif
-
-#ifdef USE_CLZ_INTRINSIC
-#if defined(_MSC_VER) && !defined(__clang__)
-#define JPEG_NBITS_NONZERO(x)  (32 - _CountLeadingZeros(x))
-#else
-#define JPEG_NBITS_NONZERO(x)  (32 - __builtin_clz(x))
-#endif
-#define JPEG_NBITS(x)          (x ? JPEG_NBITS_NONZERO(x) : 0)
-#else
-#include "jpeg_nbits_table.h"
-#define JPEG_NBITS(x)          (jpeg_nbits_table[x])
-#define JPEG_NBITS_NONZERO(x)  JPEG_NBITS(x)
-#endif
+#include "jpeg_nbits.h"
 
 
 /* Expanded entropy encoder object for Huffman encoding.