[AMDGPU] Correctly determine the toolchain linker (#89803)

Summary:
The AMDGPU toolchain simply took the short name to get the link job
instead of using the common utilities that respect options like
`-fuse-ld`. Any linker that isn't `ld.lld` will fail, however we should
be able to override it.

Author: jhuber6

clang/lib/Driver/ToolChains/AMDGPU.cpp

@@ -617,8 +617,7 @@ void amdgpu::Linker::ConstructJob(Compilation &C, const JobAction &JA,
                                   const InputInfoList &Inputs,
                                   const ArgList &Args,
                                   const char *LinkingOutput) const {
-
-  std::string Linker = getToolChain().GetProgramPath(getShortName());
+  std::string Linker = getToolChain().GetLinkerPath();
   ArgStringList CmdArgs;
   CmdArgs.push_back("--no-undefined");
   CmdArgs.push_back("-shared");

clang/test/Driver/amdgpu-toolchain.c

@@ -24,3 +24,9 @@
 // RUN:   -L. -fconvergent-functions %s 2>&1 | FileCheck -check-prefix=MCPU %s
 // LTO: clang{{.*}} "-flto=full"{{.*}}"-fconvergent-functions"
 // MCPU: ld.lld{{.*}}"-L."{{.*}}"-plugin-opt=mcpu=gfx906"
+
+// We do not suppor the BFD linker, but we should be able to override the
+// default even if it will error during linking.
+// RUN: %clang -### --target=amdgcn-amd-amdhsa -mcpu=gfx906 -nogpulib \
+// RUN:   -fuse-ld=bfd %s 2>&1 | FileCheck -check-prefixes=LD %s
+// LD: ld.bfd"