Revert "Remove useless define from clzdi2 and ctzdi2"

This reverts commit 1d54852.

Author: mintsuki

clzdi2.c

@@ -14,6 +14,16 @@
 
 // Returns: the number of leading 0-bits
 
+#if !defined(__clang__) &&                                                     \
+    ((defined(__sparc__) && defined(__arch64__)) || defined(__mips64) ||       \
+     (defined(__riscv) && __SIZEOF_POINTER__ >= 8))
+// On 64-bit architectures with neither a native clz instruction nor a native
+// ctz instruction, gcc resolves __builtin_clz to __clzdi2 rather than
+// __clzsi2, leading to infinite recursion.
+#define __builtin_clz(a) __clzsi2(a)
+extern int __clzsi2(si_int);
+#endif
+
 // Precondition: a != 0
 
 COMPILER_RT_ABI int __clzdi2(di_int a) {

ctzdi2.c

@@ -14,6 +14,16 @@
 
 // Returns: the number of trailing 0-bits
 
+#if !defined(__clang__) &&                                                     \
+    ((defined(__sparc__) && defined(__arch64__)) || defined(__mips64) ||       \
+     (defined(__riscv) && __SIZEOF_POINTER__ >= 8))
+// On 64-bit architectures with neither a native clz instruction nor a native
+// ctz instruction, gcc resolves __builtin_ctz to __ctzdi2 rather than
+// __ctzsi2, leading to infinite recursion.
+#define __builtin_ctz(a) __ctzsi2(a)
+extern int __ctzsi2(si_int);
+#endif
+
 // Precondition: a != 0
 
 COMPILER_RT_ABI int __ctzdi2(di_int a) {