Update to 2020 compilers (#1733)

Also link to libatomic in cmake and for Raspbian only in gradle.

shared/config.gradle

@@ -22,6 +22,10 @@ toolchainsPlugin.crossCompilers.named(nativeUtils.wpi.platforms.roborio) {
     optional.set(true)
 }
 
+nativeUtils.configurePlatform(nativeUtils.wpi.platforms.raspbian) {
+  linker.args << '-latomic'
+}
+
 model {
     components {
         all {