disas/libvixl/a64/instructions-a64.h: Remove unused constants

The instructions-a64.h header defines a number of floating point
constants whose initializers are function calls. gcc 5 will warn
if these constants are not used by the C or C++ file which includes
the header, because they imply a runtime cost. Since for the files
QEMU uses from libvixl we don't use these constants at all, just
remove them.

Upstream intend to fix these by shifting to an 'extern const' in
the header plus definition in a suitable source file, so we can
drop this patch when we sync with the upcoming libvixl 1.7.

The related compiling error:

    CXX   disas/arm-a64.o
  In file included from /upstream/qemu/disas/libvixl/a64/disasm-a64.h:32:0,
                   from disas/arm-a64.cc:20:
  disas/libvixl/a64/instructions-a64.h:98:13: error: 'vixl::kFP32PositiveInfinity' defined but not used [-Werror=unused-variable]
   const float kFP32PositiveInfinity = rawbits_to_float(0x7f800000);
               ^
  disas/libvixl/a64/instructions-a64.h:99:13: error: 'vixl::kFP32NegativeInfinity' defined but not used [-Werror=unused-variable]
   const float kFP32NegativeInfinity = rawbits_to_float(0xff800000);
               ^
  disas/libvixl/a64/instructions-a64.h:100:14: error: 'vixl::kFP64PositiveInfinity' defined but not used [-Werror=unused-variable]
   const double kFP64PositiveInfinity =
                ^
  disas/libvixl/a64/instructions-a64.h:102:14: error: 'vixl::kFP64NegativeInfinity' defined but not used [-Werror=unused-variable]
   const double kFP64NegativeInfinity =
                ^
  disas/libvixl/a64/instructions-a64.h:107:21: error: 'vixl::kFP64SignallingNaN' defined but not used [-Werror=unused-variable]
   static const double kFP64SignallingNaN =
                       ^
  disas/libvixl/a64/instructions-a64.h:109:20: error: 'vixl::kFP32SignallingNaN' defined but not used [-Werror=unused-variable]
   static const float kFP32SignallingNaN = rawbits_to_float(0x7f800001);
                      ^
  disas/libvixl/a64/instructions-a64.h:112:21: error: 'vixl::kFP64QuietNaN' defined but not used [-Werror=unused-variable]
   static const double kFP64QuietNaN =
                       ^
  disas/libvixl/a64/instructions-a64.h:114:20: error: 'vixl::kFP32QuietNaN' defined but not used [-Werror=unused-variable]
   static const float kFP32QuietNaN = rawbits_to_float(0x7fc00001);
                      ^
  disas/libvixl/a64/instructions-a64.h:117:21: error: 'vixl::kFP64DefaultNaN' defined but not used [-Werror=unused-variable]
   static const double kFP64DefaultNaN =
                       ^
  disas/libvixl/a64/instructions-a64.h:119:20: error: 'vixl::kFP32DefaultNaN' defined but not used [-Werror=unused-variable]
   static const float kFP32DefaultNaN = rawbits_to_float(0x7fc00000);
                      ^
  cc1plus: all warnings being treated as errors
  make: *** [disas/arm-a64.o] Error 1

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
[PMM: Rewrote the commit message a little]
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
