fuzz, test x86_emulator: disable sse before including always_inline fns

fuzz, test x86_emulator: disable sse before including always_inline fns

Workaround for compiler rejection of SSE-using always_inlines defined before
SSE is disabled.

Compiling with _FORTIFY_SOURCE or higher levels of optimization enabled
will always_inline several library fns (memset, memcpy, ...)
(with gcc 8.2.0 and glibc 2.28).

In fuzz and x86_emulator test, the compiler is instructed not
to generate SSE instructions via: #pragma GCC target("no-sse")
because those registers are needed for use by the workload.

The combination above causes compilation failure as the inline functions
use those instructions. This is resolved by reordering the inclusion of
<stdio.h> and <string.h> to after the pragma disabling SSE generation.

It would be preferable to locate the no-sse pragma within x86-emulate.h at the
top of the file, prior to including any other headers; unfortunately doing so
before <stdlib.h> causes compilation failure due to declaration of 'atof' with:
  "SSE register return with SSE disabled".
Fortunately there is no (known) current dependency on any always_inline
SSE-inclined function declared in <stdlib.h> or any of its dependencies, so the
pragma is therefore issued immediately after inclusion of <stdlib.h> with a
comment introduced to explain its location there.

Add compile-time checks for unwanted prior inclusion of <string.h> and
<stdio.h>, which are the two headers that provide the library functions that
are handled with wrappers and listed within "x86-emulate.h" as ones "we think
might access any of the FPU state".
* Use standard-defined "EOF" macro to detect prior <stdio.h> inclusion.
* Use "_STRING_H" (non-standardized guard macro) as best-effort
  for detection of prior <string.h> inclusion. This is non-universally
  viable but will provide error output on common GLIBC systems, so
  provides some defensive coverage.

Adds conditional #include <stdio.h> to x86-emulate.h because fwrite, printf,
etc. are referenced when WRAP has been defined.

Signed-off-by: Christopher Clark <christopher.clark6@baesystems.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>