Update Generic Unix linker selection

Not all generic-"unix" environments have the Gold linker available to
them, and in some cases, the vendor of the toolchain may provide their
own linker. In these cases, the driver should be internally consistent
with the toolchain that it is shipped with.

Now that we have the clang-linker, we can lean on the linker selection
in the clang-linker to determine a default linker. If the clang-linker,
and thus, the swift compiler driver, are part of a specific toolchain,
that clang-linker should be built for that platform with the appropriate
linker defaults set. If someone overrides the linker with
`-use-ld`, we should still honour that, but should otherwise be
consistent with the appropriate toolchain linker.

Fixes: rdar://123061492

Author: etcwilde

Sources/SwiftDriver/Jobs/DarwinToolchain+LinkerSupport.swift

@@ -177,8 +177,8 @@ extension DarwinToolchain {
       }
     }
 
-    if let arg = parsedOptions.getLastArgument(.useLd) {
-      commandLine.appendFlag("-fuse-ld=\(arg.asSingle)")
+    if let arg = parsedOptions.getLastArgument(.useLd)?.asSingle {
+      commandLine.appendFlag("-fuse-ld=\(arg)")
     }
 
     if let arg = parsedOptions.getLastArgument(.ldPath)?.asSingle {

Sources/SwiftDriver/Jobs/GenericUnixToolchain+LinkerSupport.swift

@@ -16,29 +16,6 @@ import func TSCBasic.lookupExecutablePath
 import struct TSCBasic.AbsolutePath
 
 extension GenericUnixToolchain {
-  private func defaultLinker(for targetTriple: Triple) -> String? {
-    if targetTriple.os == .openbsd || targetTriple.os == .freeBSD ||
-         targetTriple.environment == .android ||
-         targetTriple.isFullyStaticLinux {
-      return "lld"
-    }
-
-    switch targetTriple.arch {
-    case .arm, .aarch64, .armeb, .thumb, .thumbeb:
-      // BFD linker has issues wrt relocation of the protocol conformance
-      // section on these targets, it also generates COPY relocations for
-      // final executables, as such, unless specified, we default to gold
-      // linker.
-      return "gold"
-    case .x86, .x86_64, .ppc64, .ppc64le, .systemz:
-      // BFD linker has issues wrt relocations against protected symbols.
-      return "gold"
-    default:
-      // Otherwise, use the default BFD linker.
-      return ""
-    }
-  }
-
   private func majorArchitectureName(for triple: Triple) -> String {
     // The concept of a "major" arch name only applies to Linux triples
     guard triple.os == .linux else { return triple.archName }
@@ -71,35 +48,11 @@ extension GenericUnixToolchain {
       commandLine.appendFlag("-shared")
       fallthrough
     case .executable:
-        // Select the linker to use.
-      var linker: String?
-      if let arg = parsedOptions.getLastArgument(.useLd) {
-        linker = arg.asSingle
+      // Select the linker to use.
+      if let arg = parsedOptions.getLastArgument(.useLd)?.asSingle {
+        commandLine.appendFlag("--fuse-ld=\(arg)")
       } else if lto != nil {
-        linker = "lld"
-      } else {
-        linker = defaultLinker(for: targetTriple)
-      }
-
-      if let linker = linker {
-        #if os(Haiku)
-        // For now, passing -fuse-ld on Haiku doesn't work as swiftc doesn't
-        // recognise it. Passing -use-ld= as the argument works fine.
-        commandLine.appendFlag("-use-ld=\(linker)")
-        #else
-        commandLine.appendFlag("-fuse-ld=\(linker)")
-        #endif
-        // Starting with lld 13, Swift stopped working with the lld
-        // --gc-sections implementation for ELF, unless -z nostart-stop-gc is
-        // also passed to lld:
-        //
-        // https://reviews.llvm.org/D96914
-        if linker == "lld" || linker.hasSuffix("ld.lld") {
-          commandLine.appendFlag(.Xlinker)
-          commandLine.appendFlag("-z")
-          commandLine.appendFlag(.Xlinker)
-          commandLine.appendFlag("nostart-stop-gc")
-        }
+        commandLine.appendFlag("--fuse-ld=lld")
       }
 
       if let arg = parsedOptions.getLastArgument(.ldPath)?.asSingle {