[Clang][Driver] Override Generic_ELF::buildLinker() to avoid calling gcc to link

This change primarily makes it so that when targeting freestanding/unknown
OSes, the driver will not use `gcc` to link, but rather will link using
the chosen linker (`-fuse-ld=...`) directly.

If Clang is to be a cross compiler, there is no reason to assume that the
host's gcc is in any way capable of handling the linkage of programs for
targets that do not match the host's triple, especially for freestanding
targets.

Author: mintsuki

clang/lib/Driver/ToolChains/Gnu.cpp

@@ -3375,3 +3375,5 @@ void Generic_ELF::addClangTargetOptions(const ArgList &DriverArgs,
                           options::OPT_fno_use_init_array, true))
     CC1Args.push_back("-fno-use-init-array");
 }
+
+Tool *Generic_ELF::buildLinker() const { return new tools::gnutools::Linker(*this); }

clang/lib/Driver/ToolChains/Gnu.h

@@ -382,6 +382,9 @@ class LLVM_LIBRARY_VISIBILITY Generic_ELF : public Generic_GCC {
   }
 
   virtual void addExtraOpts(llvm::opt::ArgStringList &CmdArgs) const {}
+
+protected:
+  Tool *buildLinker() const override;
 };
 
 } // end namespace toolchains

clang/test/Driver/baremetal-ld.c

@@ -4,3 +4,14 @@
 
 // RUN: %clang -### --target=armv7-unknown-none-eabi -mcpu=cortex-m4 --sysroot= -fuse-ld=ld -flto -O3 %s 2>&1 | FileCheck --check-prefix=LTO %s
 // LTO: {{".*ld.*"}} {{.*}} "-plugin-opt=mcpu=cortex-m4" "-plugin-opt=O3"
+
+// Ensure that, for freestanding -none targets, the linker does not call into gcc.
+// We do this by checking if clang is trying to pass "-fuse-ld=bfd" to the linker command.
+// RUN: %clang -### --target=aarch64-unknown-none-elf -fuse-ld=bfd %s 2>&1 | FileCheck --check-prefix=FUSELDAARCH64 %s
+// FUSELDAARCH64-NOT: "-fuse-ld=bfd"
+// RUN: %clang -### --target=loongarch64-unknown-none-elf -fuse-ld=bfd %s 2>&1 | FileCheck --check-prefix=FUSELDLOONGARCH64 %s
+// FUSELDLOONGARCH64-NOT: "-fuse-ld=bfd"
+// RUN: %clang -### --target=riscv64-unknown-none-elf -fuse-ld=bfd %s 2>&1 | FileCheck --check-prefix=FUSELDRISCV64 %s
+// FUSELDRISCV64-NOT: "-fuse-ld=bfd"
+// RUN: %clang -### --target=x86_64-unknown-none-elf -fuse-ld=bfd %s 2>&1 | FileCheck --check-prefix=FUSELDX8664 %s
+// FUSELDX8664-NOT: "-fuse-ld=bfd"