[GR-11010] Store paths relative to the home for core C extensions

* So they can be debugged with GraalVM and --inspect.SourcePath=trufflerubyCheckout

Author: eregon

lib/cext/preprocess.rb

@@ -72,7 +72,8 @@ def self.patch(file, contents, directory)
       file_name = patched_file_name
     end
 
-    $stdout.puts "#line 1 \"#{file_name}\""
+    expanded_path = File.expand_path(file_name)
+    $stdout.puts "#line 1 \"#{expanded_path}\""
     $stdout.puts output
   end
 end

lib/truffle/rbconfig-for-mkmf.rb

@@ -85,17 +85,20 @@
 
 cext_dir = "#{RbConfig::CONFIG['libdir']}/cext"
 
+expanded = RbConfig::CONFIG
+mkconfig = RbConfig::MAKEFILE_CONFIG
+
 if Truffle::Boot.get_option 'building.core.cexts'
   ruby_home = Truffle::Boot.ruby_home
   link_o_files = "#{ruby_home}/src/main/c/cext/ruby.o #{ruby_home}/src/main/c/sulongmock/sulongmock.o"
-  RbConfig::MAKEFILE_CONFIG['CPPFLAGS'] = "-DSULONG_POLYGLOT_H='\"#{ENV.fetch('SULONG_POLYGLOT_H')}\"'"
+  relative_debug_paths = "-fdebug-prefix-map=#{ruby_home}=."
+  polyglot_h = "-DSULONG_POLYGLOT_H='\"#{ENV.fetch('SULONG_POLYGLOT_H')}\"'"
+  mkconfig['CPPFLAGS'] = "#{relative_debug_paths} #{polyglot_h}"
+  expanded['CPPFLAGS'] = mkconfig['CPPFLAGS']
 else
   link_o_files = "#{cext_dir}/ruby.o #{cext_dir}/sulongmock.o"
 end
 
-expanded = RbConfig::CONFIG
-mkconfig = RbConfig::MAKEFILE_CONFIG
-
 common = {
   'CC' => cc,
   'CPP' => cc,