修复clang开启-g编译不过的问题

kernel/src/ctype/isalpha.c

+#include <ctype.h>
+#undef isalpha
+
+int isalpha(int c)
+{
+	return ((unsigned)c|32)-'a' < 26;
+}
+
+int __isalpha_l(int c, locale_t l)
+{
+	return isalpha(c);
+}
+
+weak_alias(__isalpha_l, isalpha_l);