ctype.h: remove duplicate isdigit() helper

gcc warns a few thousand times about the isdigit() shadow:

include/linux/ctype.h:26:19: warning: declaration of 'isdigit' shadows a built-in function [-Wshadow]

As there is already a compiler builtin, just use that, and make
it clear we do that by defining a macro.  Unfortunately, clang
does not have the isdigit() builtin, so this has to be conditional.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>

include/linux/compiler_types.h

@@ -64,6 +64,17 @@ static inline void __chk_io_ptr(const volatile void __iomem *ptr) { }
 /* Attributes */
 #include <linux/compiler_attributes.h>
 
+/* Builtins */
+
+/*
+ * __has_builtin is supported on gcc >= 10, clang >= 3 and icc >= 21.
+ * In the meantime, to support gcc < 10, we implement __has_builtin
+ * by hand.
+ */
+#ifndef __has_builtin
+#define __has_builtin(x) (0)
+#endif
+
 /* Compiler specific macros. */
 #ifdef __clang__
 #include <linux/compiler-clang.h>

include/linux/ctype.h

@@ -2,6 +2,8 @@
 #ifndef _LINUX_CTYPE_H
 #define _LINUX_CTYPE_H
 
+#include <linux/compiler.h>
+
 /*
  * NOTE! This ctype does not handle EOF like the standard C
  * library is required to.
@@ -23,10 +25,6 @@ extern const unsigned char _ctype[];
 #define isalnum(c)	((__ismask(c)&(_U|_L|_D)) != 0)
 #define isalpha(c)	((__ismask(c)&(_U|_L)) != 0)
 #define iscntrl(c)	((__ismask(c)&(_C)) != 0)
-static inline int isdigit(int c)
-{
-	return '0' <= c && c <= '9';
-}
 #define isgraph(c)	((__ismask(c)&(_P|_U|_L|_D)) != 0)
 #define islower(c)	((__ismask(c)&(_L)) != 0)
 #define isprint(c)	((__ismask(c)&(_P|_U|_L|_D|_SP)) != 0)
@@ -39,6 +37,15 @@ static inline int isdigit(int c)
 #define isascii(c) (((unsigned char)(c))<=0x7f)
 #define toascii(c) (((unsigned char)(c))&0x7f)
 
+#if __has_builtin(__builtin_isdigit)
+#define  isdigit(c) __builtin_isdigit(c)
+#else
+static inline int isdigit(int c)
+{
+	return '0' <= c && c <= '9';
+}
+#endif
+
 static inline unsigned char __tolower(unsigned char c)
 {
 	if (isupper(c))