Fix Neon SIMD build issues with Visual Studio

- Use the _M_ARM and _M_ARM64 macros provided by Visual Studio for
  compile-time detection of Arm builds, since __arm__ and __aarch64__
  are only present in GNU-compatible compilers.
- Neon/intrinsics: Use the _CountLeadingZeros() and
  _CountLeadingZeros64() intrinsics provided by Visual Studio, since
  __builtin_clz() and __builtin_clzl() are only present in
  GNU-compatible compilers.
- Neon/intrinsics: Since Visual Studio does not support static vector
  initialization, replace static initialization of Neon vectors with the
  appropriate intrinsics.  Compared to the static initialization
  approach, this produces identical assembly code with both GCC and
  Clang.
- Neon/intrinsics: Since Visual Studio does not support inline assembly
  code, provide alternative code paths for Visual Studio whenever inline
  assembly is used.
- Build: Set FLOATTEST appropriately for AArch64 Visual Studio builds
  (Visual Studio does not emit fused multiply-add [FMA] instructions by
  default for such builds.)
- Neon/intrinsics: Move temporary buffer allocation outside of nested
  loops.  Since Visual Studio configures Arm builds with a relatively
  small amount of stack memory, attempting to allocate those buffers
  within the inner loops caused a stack overflow.

Closes #461
Closes #475

simd/arm/jchuff.h

@@ -17,7 +17,7 @@
  * but must not be updated permanently until we complete the MCU.
  */
 
-#if defined(__aarch64__)
+#if defined(__aarch64__) || defined(_M_ARM64)
 #define BIT_BUF_SIZE  64
 #else
 #define BIT_BUF_SIZE  32
@@ -54,7 +54,25 @@ typedef struct {
  * directly to the output buffer.  Otherwise, use the EMIT_BYTE() macro to
  * encode 0xFF as 0xFF 0x00.
  */
-#if defined(__aarch64__)
+#if defined(__aarch64__) || defined(_M_ARM64)
+
+#if defined(_MSC_VER) && !defined(__clang__)
+#define SPLAT() { \
+  buffer[0] = (JOCTET)(put_buffer >> 56); \
+  buffer[1] = (JOCTET)(put_buffer >> 48); \
+  buffer[2] = (JOCTET)(put_buffer >> 40); \
+  buffer[3] = (JOCTET)(put_buffer >> 32); \
+  buffer[4] = (JOCTET)(put_buffer >> 24); \
+  buffer[5] = (JOCTET)(put_buffer >> 16); \
+  buffer[6] = (JOCTET)(put_buffer >>  8); \
+  buffer[7] = (JOCTET)(put_buffer      ); \
+}
+#else
+#define SPLAT() { \
+  __asm__("rev %x0, %x1" : "=r"(put_buffer) : "r"(put_buffer)); \
+  *((uint64_t *)buffer) = put_buffer; \
+}
+#endif
 
 #define FLUSH() { \
   if (put_buffer & 0x8080808080808080 & ~(put_buffer + 0x0101010101010101)) { \
@@ -67,14 +85,27 @@ typedef struct {
     EMIT_BYTE(put_buffer >>  8) \
     EMIT_BYTE(put_buffer      ) \
   } else { \
-    __asm__("rev %x0, %x1" : "=r"(put_buffer) : "r"(put_buffer)); \
-    *((uint64_t *)buffer) = put_buffer; \
+    SPLAT() \
     buffer += 8; \
   } \
 }
 
 #else
 
+#if defined(_MSC_VER) && !defined(__clang__)
+#define SPLAT() { \
+  buffer[0] = (JOCTET)(put_buffer >> 24); \
+  buffer[1] = (JOCTET)(put_buffer >> 16); \
+  buffer[2] = (JOCTET)(put_buffer >>  8); \
+  buffer[3] = (JOCTET)(put_buffer      ); \
+}
+#else
+#define SPLAT() { \
+  __asm__("rev %0, %1" : "=r"(put_buffer) : "r"(put_buffer)); \
+  *((uint32_t *)buffer) = put_buffer; \
+}
+#endif
+
 #define FLUSH() { \
   if (put_buffer & 0x80808080 & ~(put_buffer + 0x01010101)) { \
     EMIT_BYTE(put_buffer >> 24) \
@@ -82,8 +113,7 @@ typedef struct {
     EMIT_BYTE(put_buffer >>  8) \
     EMIT_BYTE(put_buffer      ) \
   } else { \
-    __asm__("rev %0, %1" : "=r"(put_buffer) : "r"(put_buffer)); \
-    *((uint32_t *)buffer) = put_buffer; \
+    SPLAT() \
     buffer += 4; \
   } \
 }