Allow overriding bzero libcall from command line

[gchatelet]

Summary

bzero libcall is disabled except on Darwin version 10+.
This patch enables the use of bzero (especially on X86 when zeroing out memory) when user can provide a fast implementation.

[gchatelet]

> cat test.cc
#include <cstring>

void foo(char* ptr, unsigned size) {
    memset(ptr, 0, size);
}

>  /tmp/llvm-project_dbg_compiled-with-clang/bin/clang++ test.cc --S -O3 -o-
# %bb.0:                                # %entry
        movl    %esi, %edx
        xorl    %esi, %esi
        jmp     memset                  # TAILCALL

>  /tmp/llvm-project_dbg_compiled-with-clang/bin/clang++ test.cc -mllvm --override-libcall-bzero=bzero -S -O3 -o-
# %bb.0:                                # %entry
        pushq   %rax
        movl    %esi, %esi
        callq   bzero
        popq    %rax
        retq

[gchatelet]

As discussed with @courbet I'm dropping this review because we don't want to add more backend flags.

llvm/lib/CodeGen/TargetLoweringBase.cpp

JumpTableDensity("jump-table-density", cl::init(10), cl::Hidden,
                               "a normal function"));
 
 /// Minimum jump table density for -Os or -Oz functions.
 static cl::opt<unsigned> OptsizeJumpTableDensity(
     "optsize-jump-table-density", cl::init(40), cl::Hidden,
     cl::desc("Minimum density for building a jump table in "
              "an optsize function"));
 
+/// Allow overriding bzero libcall
+cl::opt<std::string>
+    OverrideBzeroLibcall("override-libcall-bzero",
+                         cl::desc("The function name for bzero"), cl::Hidden);
+
 static bool darwinHasSinCos(const Triple &TT) {
   assert(TT.isOSDarwin() && "should be called with darwin triple");
   // Don't bother with 32 bit x86.
   if (TT.getArch() == Triple::x86)
     return false;
   // Macos < 10.9 has no sincos_stret.
   if (TT.isMacOSX())
     return !TT.isMacOSXVersionLT(10, 9) && TT.isArch64Bit();
#undef HANDLE_LIBCALL
   if (TT.isPS4CPU()) {
     setLibcallName(RTLIB::SINCOS_F32, "sincosf");
     setLibcallName(RTLIB::SINCOS_F64, "sincos");
   }
 
   if (TT.isOSOpenBSD()) {
     setLibcallName(RTLIB::STACKPROTECTOR_CHECK_FAIL, nullptr);
   }
+
+  if (!OverrideBzeroLibcall.empty())
+    setLibcallName(RTLIB::BZERO, OverrideBzeroLibcall.data());
 }
 
 /// getFPEXT - Return the FPEXT_*_* value for the given types, or
 /// UNKNOWN_LIBCALL if there is none.
 RTLIB::Libcall RTLIB::getFPEXT(EVT OpVT, EVT RetVT) {
   if (OpVT == MVT::f16) {
     if (RetVT == MVT::f32)
       return FPEXT_F16_F32;