[libc] Using llvm_libc memcpy in mem* benchmarks.

[asteinhauser]

Summary

Currently the mem* benchmarks use memcpy from Glibc and memset from LLVM libc.
That's misleading and produces inconsistent results and behaviors.

This change makes Memcpy.cpp consistent with Memset.cpp:
https://github.com/llvm/llvm-project/blob/master/libc/benchmarks/Memset.cpp#L49

[sivachandra]

Ah, thanks for catching this. I seem to have missed this in https://reviews.llvm.org/D81910.

libc/benchmarks/Memcpy.cpp

struct MemcpyContext : public BenchmarkRunner {
 
   ArrayRef<StringRef> getFunctionNames() const override {
     static std::array<StringRef, 1> kFunctionNames = {"memcpy"};
     return kFunctionNames;
   }
 
   BenchmarkResult benchmark(const BenchmarkOptions &Options,
                             StringRef FunctionName, size_t Size) override {
-    FunctionPrototype Function =
-        StringSwitch<FunctionPrototype>(FunctionName).Case("memcpy", &::memcpy);
+    FunctionPrototype Function = StringSwitch<FunctionPrototype>(FunctionName)
+                                     .Case("memcpy", &__llvm_libc::memcpy);
     return llvm::libc_benchmarks::benchmark(
         Options, PP, [this, Function, Size](ParameterType p) {
           Function(DstBuffer + p.DstOffset, SrcBuffer + p.SrcOffset, Size);
           return DstBuffer + p.DstOffset;
         });
   }
 
 private: