[LLVM][LTO] Factor out RTLib calls and allow them to be dropped

Summary:
The LTO pass and LLD linker have logic in them that forces extraction
and prevent internalization of needed runtime calls. However, these
currently take all RTLibcalls into account, even if the target does not
support them. The target opts-out of a libcall if it sets its name to
nullptr. This patch pulls this logic out into a class in the header so
that LTO / lld can use it to determine if a symbol actually needs to be
kept.

This is important for targets like AMDGPU that want to be able to use
`lld` to perform the final link step, but cannot maintain the overhead
of several unused function calls.

lld/COFF/Driver.cpp

@@ -2428,9 +2428,12 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
       // file's symbol table. If any of those library functions are defined in a
       // bitcode file in an archive member, we need to arrange to use LTO to
       // compile those archive members by adding them to the link beforehand.
-      if (!ctx.bitcodeFileInstances.empty())
-        for (auto *s : lto::LTO::getRuntimeLibcallSymbols())
+      if (!ctx.bitcodeFileInstances.empty()) {
+        llvm::Triple TT(
+            ctx.bitcodeFileInstances.front()->obj->getTargetTriple());
+        for (auto *s : lto::LTO::getRuntimeLibcallSymbols(TT))
           ctx.symtab.addLibcall(s);
+      }
 
       // Windows specific -- if __load_config_used can be resolved, resolve it.
       if (ctx.symtab.findUnderscore("_load_config_used"))

lld/ELF/Driver.cpp

@@ -2883,9 +2883,11 @@ template <class ELFT> void LinkerDriver::link(opt::InputArgList &args) {
   // to, i.e. if the symbol's definition is in bitcode. Any other required
   // libcall symbols will be added to the link after LTO when we add the LTO
   // object file to the link.
-  if (!ctx.bitcodeFiles.empty())
-    for (auto *s : lto::LTO::getRuntimeLibcallSymbols())
+  if (!ctx.bitcodeFiles.empty()) {
+    llvm::Triple TT(ctx.bitcodeFiles.front()->obj->getTargetTriple());
+    for (auto *s : lto::LTO::getRuntimeLibcallSymbols(TT))
       handleLibcall(s);
+  }
 
   // Archive members defining __wrap symbols may be extracted.
   std::vector<WrappedSymbol> wrapped = addWrappedSymbols(args);

lld/wasm/Driver.cpp

@@ -1319,9 +1319,11 @@ void LinkerDriver::linkerMain(ArrayRef<const char *> argsArr) {
   // We only need to add libcall symbols to the link before LTO if the symbol's
   // definition is in bitcode. Any other required libcall symbols will be added
   // to the link after LTO when we add the LTO object file to the link.
-  if (!ctx.bitcodeFiles.empty())
-    for (auto *s : lto::LTO::getRuntimeLibcallSymbols())
+  if (!ctx.bitcodeFiles.empty()) {
+    llvm::Triple TT(ctx.bitcodeFiles.front()->obj->getTargetTriple());
+    for (auto *s : lto::LTO::getRuntimeLibcallSymbols(TT))
       handleLibcall(s);
+  }
   if (errorCount())
     return;