Revert F16C series (MR 6774)

This reverts commit 4fb2eddfdf.
This reverts commit 7a1deb16f8.
This reverts commit 2b6a172343.
This reverts commit 5af81393e4.
This reverts commit 87900afe5b.

A couple of problems were discovered after this series was merged that
cause breakage in different configurations:

   (1) It seems that using -mf16c also enables AVX, leading to SIGILL on
   platforms that do not support AVX.
   (2) Since clang only warns about unknown flags, and as I understand
   it Meson's handling in cc.has_argument() is broken, the F16C code is
   wrongly enabled when clang is used, even for example on ARM, leading
   to a compilation error.

Closes: https://gitlab.freedesktop.org/mesa/mesa/-/issues/3583
Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6969>

src/util/format/u_format_pack.py

@@ -360,7 +360,7 @@ def conversion_expr(src_channel,
 
     # Promote half to float
     if src_type == FLOAT and src_size == 16:
-        value = '_mesa_half_to_float(%s)' % value
+        value = 'util_half_to_float(%s)' % value
         src_size = 32
 
     # Special case for float <-> ubytes for more accurate results
@@ -436,7 +436,7 @@ def conversion_expr(src_channel,
             src_size = 32
 
         if dst_channel.size == 16:
-            value = '_mesa_float_to_float16_rtz(%s)' % value
+            value = 'util_float_to_half_rtz(%s)' % value
         elif dst_channel.size == 64 and src_size < 64:
             value = '(double)%s' % value
 
@@ -715,7 +715,7 @@ def generate(formats):
     print()
     print('#include "pipe/p_compiler.h"')
     print('#include "util/u_math.h"')
-    print('#include "util/half_float.h"')
+    print('#include "util/u_half.h"')
     print('#include "u_format.h"')
     print('#include "u_format_other.h"')
     print('#include "util/format_srgb.h"')