Move x86 constant area code to its own file

Move the logic to constant_area_fixups_x86.cc to keep the graph
modifications out of the code generation file.

Change-Id: I476f1fce80cb4ad38ae872b620ae58f6e52fe664
Signed-off-by: Mark Mendell <mark.p.mendell@intel.com>
diff --git a/compiler/Android.mk b/compiler/Android.mk
index 8e3b555..20c8023 100644
--- a/compiler/Android.mk
+++ b/compiler/Android.mk
@@ -66,6 +66,7 @@
 	optimizing/builder.cc \
 	optimizing/code_generator.cc \
 	optimizing/code_generator_utils.cc \
+	optimizing/constant_area_fixups_x86.cc \
 	optimizing/constant_folding.cc \
 	optimizing/dead_code_elimination.cc \
 	optimizing/graph_checker.cc \