scons: fix systrace header test

Latest compilers default checks the unused and uninitialized. Those
checks result in false negative of the header test. We fix it by
replacing the temp variable to (void *)1.

Btw, (void *)0 not works here because the function is declare with nonnull.
https://elixir.bootlin.com/glibc/latest/source/debug/execinfo.h#L38

Change-Id: I3b51faf7595b861bfbd131c0a42fd6d78a5e9698
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/64652
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>

src/sim/SConsopts

@@ -31,7 +31,7 @@ import gem5_scons
 
 with gem5_scons.Configure(main) as conf:
     if conf.CheckLibWithHeader([None, 'execinfo'], 'execinfo.h', 'C',
-            'char temp; backtrace_symbols_fd((void *)&temp, 0, 0);'):
+            'backtrace_symbols_fd((void *)1, 0, 0);'):
         conf.env['BACKTRACE_IMPL'] = 'glibc'
     else:
         conf.env['BACKTRACE_IMPL'] = 'none'