[LLVM 15][runtime] Fix compilation warning in {ftni64bitsup,utilsi64}.c; NFCI

[bryanpkc]

ftni64bitsup.c and utilsi64.c define a static function shf64 (a helper for shifting signed 64-bit values) identically, while only ftni64bitsup.c defines the unsigned verison ushf64. Both files call shf64 on various unsigned values, causing the compiler to complain. They also declare conflicting prototypes for the same functions, declare some functions as extern unnecessarily, and omit forward declarations when they are necessary. This patch fixes all these problems, and makes both files use the same definitions for shf64 and ushf64, reducing code duplication.