i386: fix assert (__builtin_cpu_supports ("x86-64") >= 0)

i386: fix assert (__builtin_cpu_supports ("x86-64") >= 0)

Similar story as PR103661, we again return a negative number
for __builtin_cpu_supports:

Documentation says:

int __builtin_cpu_supports(const char *feature)
This function returns a positive integer if the run-time CPU supports feature and returns 0 otherwise.
while we return -2147483648.

Moreover, I noticed "x86-64" is not a valid option for __builtin_cpu_is,
but for __builtin_cpu_supports.

PR target/107551

gcc/ChangeLog:

	* config/i386/i386-builtins.cc (fold_builtin_cpu): Use same path
	as for PR103661.
	* doc/extend.texi: Fix "x86-64" use.

gcc/testsuite/ChangeLog:

	* gcc.target/i386/builtin_target.c: Add more checks.