Require gold on linux and freebsd by default.

This commit just changes the default to require gold. The user can still override from the command line if they choose to.

The reason why I am making this change is to match Swift's behavior here. There
really isn't any good reason to have separate linkers compiling separate parts
of the same build.

rdar://39456714

configure

@@ -71,6 +71,8 @@ def reconfigure(config, path):
                 config.toolchain = Path(info['toolchain'])
             if 'linker' in info and info['linker'] is not None:
                 config.linker = info['linker']
+            elif config.target is not None:
+                config.linker = config.target.linker
             if 'build_directory' in info and info['build_directory'] is not None:
                 config.build_directory = Path(info['build_directory'])
             if 'intermediate_directory' in info and info['intermediate_directory'] is not None:
@@ -181,7 +183,10 @@ def main():
             config.swift_sdk = "/usr"
         config.toolchain = Path.path(args.toolchain)
         config.pkg_config = args.pkg_config
-        config.linker = args.linker
+        if args.linker is not None:
+            config.linker = args.linker
+        else:
+            config.linker = config.target.linker
         config.bootstrap_directory = Path.path(args.bootstrap)
         config.verbose = args.verbose
         if config.toolchain is not None:

lib/target.py

@@ -331,14 +331,17 @@ class Target:
     dynamic_library_suffix = ".dylib"
     static_library_prefix = "lib"
     static_library_suffix = ".a"
+    linker = None
 
     def __init__(self, triple):
         if "linux" in triple:
             self.sdk = OSType.Linux
             self.dynamic_library_suffix = ".so"
+            self.linker = "gold"
         elif "freebsd" in triple:
             self.sdk = OSType.FreeBSD
             self.dynamic_library_suffix = ".so"
+            self.linker = "gold"
         elif "windows" in triple or "win32" in triple:
             self.sdk = OSType.Win32
             self.dynamic_library_suffix = ".dll"