Enable specifying full target triples as --arch for ldc2

By simply treating all non-empty strings except for special `x86[_64]`
as `-mtriple` values.
E.g., `dub --arch=wasm32-unknown-unknown-wasm` to cross-compile to
WebAssembly. This dub switch is already used when probing the compiler,
so the build platform detection works as expected.

Also add `wasm` as detected platform and use the .wasm executable file
extension (for ldc2).

This fixes dlang#1749 when moving the `-mtriple` option from the dub config
file to the dub command line.

source/dub/compilers/ldc.d

@@ -80,10 +80,10 @@ config    /etc/ldc2.conf (x86_64-pc-linux-gnu)
 	{
 		string[] arch_flags;
 		switch (arch_override) {
-			default: throw new Exception("Unsupported architecture: "~arch_override);
 			case "": break;
 			case "x86": arch_flags = ["-march=x86"]; break;
 			case "x86_64": arch_flags = ["-march=x86-64"]; break;
+			default: arch_flags = ["-mtriple=" ~ arch_override]; break;
 		}
 		settings.addDFlags(arch_flags);
 
@@ -176,6 +176,8 @@ config    /etc/ldc2.conf (x86_64-pc-linux-gnu)
 			case TargetType.executable:
 				if (platform.platform.canFind("windows"))
 					return settings.targetName ~ ".exe";
+				else if (platform.platform.canFind("wasm"))
+					return settings.targetName ~ ".wasm";
 				else return settings.targetName;
 			case TargetType.library:
 			case TargetType.staticLibrary:

source/dub/platform.d

@@ -42,6 +42,7 @@ enum string platformCheck = q{
 	version(Android) ret ~= "android";
 	version(Cygwin) ret ~= "cygwin";
 	version(MinGW) ret ~= "mingw";
+	version(WebAssembly) ret ~= "wasm";
 	return ret;
 };