Modularize signal.h on FreeBSD

This nudges the Glibc modulemap just enough to get SwiftPM and later
targets building. Without it, Swift tries to import the signal hander
APIs through CDispatch instead of Glibc. CDispatch shouldn't be directly
imported, so recommending that is nonsensical.

This is not a full solution for all headers, but meant to perturb the
module machinery sufficiently that it pulls signal.h into glibc
correctly.

Works around:
```
Swift-Project/swiftpm/Sources/Basics/Cancellator.swift:79:24:
error: property '__sigaction_u' is not available due to missing import of defining module 'CDispatch' [#MemberImportVisibility]
```

Author: etcwilde

stdlib/public/Platform/glibc.modulemap.gyb

@@ -49,6 +49,14 @@ module SwiftGlibc [system] {
   export *
 }
 
+% if CMAKE_SDK == "FREEBSD":
+module signal_h [system] {
+  config_macros _BSD_VISIBLE, _POSIX_VISIBLE
+  header "signal.h"
+  export *
+}
+% end
+
 module CUUID [system] {
   header "uuid/uuid.h"
   link "uuid"