[embedded] Add %linux_rng_support_c_if_needed lit substitution

Author: kubamracek

test/embedded/linkage-mergeable4.swift

@@ -3,7 +3,7 @@
 
 // RUN: %target-swift-frontend -mergeable-symbols -num-threads 2 -O -c -emit-module -o %t/MyModule.o             %t/MyModule.swift                   -enable-experimental-feature Embedded -parse-as-library
 // RUN: %target-swift-frontend -mergeable-symbols -num-threads 2 -O -c              -o %t/MainA.o -o %t/MainB.o  %t/MainA.swift %t/MainB.swift -I %t -enable-experimental-feature Embedded -parse-as-library
-// RUN: %target-clang %t/MainA.o %t/MainB.o %t/MyModule.o -o %t/a.out
+// RUN: %target-clang %linux_rng_support_c_if_needed %t/MainA.o %t/MainB.o %t/MyModule.o -o %t/a.out
 // RUN: %target-run %t/a.out | %FileCheck %s
 
 // REQUIRES: swift_in_compiler

test/embedded/lit.local.cfg

@@ -28,3 +28,9 @@ if config.available_features.intersection(set(supported_test_os_list)) == set():
 config.environment = dict(config.environment)
 if 'SWIFT_USE_OLD_DRIVER' in config.environment: del config.environment['SWIFT_USE_OLD_DRIVER']
 if 'SWIFT_AVOID_WARNING_USING_OLD_DRIVER' in config.environment: del config.environment['SWIFT_AVOID_WARNING_USING_OLD_DRIVER']
+
+# (5) Provide some useful substitutions to simplify writing tests that work across platforms (macOS, Linux, etc.)
+if "OS=linux-gnu" in config.available_features:
+  config.substitutions.append(("%linux_rng_support_c_if_needed", "%S/Inputs/linux-rng-support.c"))
+else:
+  config.substitutions.append(("%linux_rng_support_c_if_needed", ""))