[embedded] Use @_extern(c) instead of @_silgen_name on arc4random_buf

Author: kubamracek

stdlib/public/core/EmbeddedRuntime.swift

@@ -275,10 +275,9 @@ public func swift_deletedMethodError() -> Never {
 public func swift_willThrow() throws {
 }
 
-@_silgen_name("arc4random_buf")
+@_extern(c, "arc4random_buf")
 public func arc4random_buf(buf: UnsafeMutableRawPointer, nbytes: Int)
 
-@_silgen_name("swift_stdlib_random")
 public func swift_stdlib_random(_ buf: UnsafeMutableRawPointer, _ nbytes: Int) {
   arc4random_buf(buf: buf, nbytes: nbytes)
 }