build: correct the DT_SONAME

When ninja is invoked from a subdirectory it will convert the path to a
relative path, and that resulted in incorrect test results.  This
ensures that we get the correct soname.

Author: compnerd

Diff for: cmake/modules/SwiftSupport.cmake

@@ -136,7 +136,7 @@ function(add_swift_target target)
     if(CMAKE_SYSTEM_NAME STREQUAL Windows OR CMAKE_SYSTEM_NAME STREQUAL Darwin)
       set(emit_library -emit-library)
     else()
-      set(emit_library -emit-library -Xlinker -soname -Xlinker ${AST_OUTPUT})
+      set(emit_library -emit-library -Xlinker -soname -Xlinker ${CMAKE_SHARED_LIBRARY_PREFIX}${target}${CMAKE_SHARED_LIBRARY_SUFFIX})
     endif()
   endif()
   if(NOT AST_LIBRARY OR library_kind STREQUAL SHARED)