[builtins] Use __builtin_clzll for 64-bit types (#99874)

Summary:
This addresses the issue with `__LP64__` not being defined for targets
with 32-bit pointers but 64-bit longs, resulting in worse codegen.

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Differential Revision: https://phabricator.intern.facebook.com/D60251560

Author: s-barannikov

compiler-rt/lib/builtins/fp_extend.h

@@ -37,16 +37,7 @@ static const int srcSigFracBits = 52;
 // srcBits - srcSigFracBits - 1
 static const int srcExpBits = 11;
 
-static inline int src_rep_t_clz_impl(src_rep_t a) {
-#if defined __LP64__
-  return __builtin_clzl(a);
-#else
-  if (a & REP_C(0xffffffff00000000))
-    return clzsi(a >> 32);
-  else
-    return 32 + clzsi(a & REP_C(0xffffffff));
-#endif
-}
+static inline int src_rep_t_clz_impl(src_rep_t a) { return __builtin_clzll(a); }
 #define src_rep_t_clz src_rep_t_clz_impl
 
 #elif defined SRC_80

compiler-rt/lib/builtins/fp_lib.h

@@ -58,16 +58,7 @@ typedef double fp_t;
 #define REP_C UINT64_C
 #define significandBits 52
 
-static __inline int rep_clz(rep_t a) {
-#if defined __LP64__
-  return __builtin_clzl(a);
-#else
-  if (a & REP_C(0xffffffff00000000))
-    return clzsi(a >> 32);
-  else
-    return 32 + clzsi(a & REP_C(0xffffffff));
-#endif
-}
+static inline int rep_clz(rep_t a) { return __builtin_clzll(a); }
 
 #define loWord(a) (a & 0xffffffffU)
 #define hiWord(a) (a >> 32)