[embedded] Fix linux-rng-support.c compiling in C++ mode, make it C instead

Author: kubamracek

Diff for: test/embedded/Inputs/linux-rng-support.c

@@ -5,14 +5,8 @@
 
 #ifdef __linux__ 
 
-#ifdef __cplusplus
-extern "C"
-#endif
 ssize_t getrandom(void *buf, size_t len, unsigned int flags);
 
-#ifdef __cplusplus
-extern "C"
-#endif
 void arc4random_buf(void *buf, size_t nbytes);
 
 void arc4random_buf(void *buf, size_t nbytes) {

Diff for: test/embedded/lit.local.cfg

@@ -31,6 +31,6 @@ if 'SWIFT_AVOID_WARNING_USING_OLD_DRIVER' in config.environment: del config.envi
 
 # (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"))
+  config.substitutions.append(("%linux_rng_support_c_if_needed", "-x c %S/Inputs/linux-rng-support.c"))
 else:
   config.substitutions.append(("%linux_rng_support_c_if_needed", ""))