Fix handling of ldscript.so files.

--- a/lib/ffi/library.rb
+++ b/lib/ffi/library.rb
@@ -114,7 +114,7 @@
 
             rescue Exception => ex
               ldscript = false
-              if ex.message =~ /(([^ \t()])+\.so([^ \t:()])*):([ \t])*(invalid ELF header|file too short|invalid file format)/
+              if ex.message =~ /(([^ \t()])+\.so([^ \t:()])*):([ \t])*(invalid ELF header|file too short|invalid file format|Exec format error)/
                 if File.binread($1) =~ /(?:GROUP|INPUT) *\( *([^ \)]+)/
                   libname = $1
                   ldscript = true