Use LinuxMain on all platforms except Darwin (swiftlang#2734)

* Guard planLinuxMain only on Darwin platforms

LinuxMain is also used to run tests on WebAssembly/WASI (and potentially on Android, Windows etc)

* Append from linuxMainMap w/ all non-Darwin triples

Sources/Build/BuildPlan.swift

@@ -1205,7 +1205,7 @@ public class BuildPlan {
         _ buildParameters: BuildParameters,
         _ graph: PackageGraph
     ) throws -> [(ResolvedProduct, SwiftTargetBuildDescription)] {
-        guard buildParameters.triple.isLinux() else {
+        guard !buildParameters.triple.isDarwin() else {
             return []
         }
 
@@ -1530,7 +1530,7 @@ public class BuildPlan {
             }
         }
 
-        if buildParameters.triple.isLinux() {
+        if !buildParameters.triple.isDarwin() {
             if product.type == .test {
                 linuxMainMap[product].map{ staticTargets.append($0) }
             }