Fix gdb.arch/i386-sse.exp with clang

Fix gdb.arch/i386-sse.exp with clang

gdb.arch/i386-sse.exp fails to run with clang, because of:

 gdb compile failed, /gdbtest/src/gdb/testsuite/gdb.arch/i386-sse.c:56:40: warning:
       passing 'int *' to parameter of type 'unsigned int *' converts between
       pointers to integer types with different sign [-Wpointer-sign]
   if (!x86_cpuid (1, NULL, NULL, NULL, &edx))
                                        ^~~~
 /gdbtest/src/gdb/testsuite/../nat/x86-cpuid.h:35:41: note: passing
       argument to parameter '__edx' here
             unsigned int *__ecx, unsigned int *__edx)
                                               ^
 1 warning generated.

Fix it by declaring edx unsigned.

gdb/testsuite/ChangeLog:

	* gdb.arch/i386-sse.c (have_sse) <edx>: Make unsigned.