<linux/overflow.h>: Don't use __has_builtin().

GCC only added support for __has_builtin in GCC 10.  However, all
supported versions of GCC and clang include these builtins so just use
them unconditionally.

This fixes the build with GCC 9.

Reviewed by:	manu, hselasky, imp
Differential Revision:	https://reviews.freebsd.org/D31942

sys/compat/linuxkpi/common/include/linux/overflow.h

@@ -34,18 +34,9 @@
 #include <sys/stdint.h>
 #include <sys/types.h>
 
-#ifndef	__has_builtin
-#define	__has_builtin(x)	0
-#endif
-
-#if __has_builtin(__builtin_add_overflow)
 #define check_add_overflow(a, b, c)		\
 	__builtin_add_overflow(a, b, c)
-#else
-#error "Compiler does not support __builtin_add_overflow"
-#endif
 
-#if __has_builtin(__builtin_mul_overflow)
 #define check_mul_overflow(a, b, c)	\
 	__builtin_mul_overflow(a, b, c)
 
@@ -58,8 +49,5 @@ array_size(size_t x, size_t y)
 		retval = SIZE_MAX;
 	return (retval);
 }
-#else
-#error "Compiler does not support __builtin_mul_overflow"
-#endif
 
 #endif	/* __LINUX_OVERFLOW_H__ */