[gcc] Fix lea splitters for ILP32 data model

This is similar to http://codereview.chromium.org/3187016 and several previous changes.

Fixed lea splitters to disallow PIC symbol references where immediate operand is needed. Now all specs cpu2000 compile with -fPIC.

Hope the process of making these fixes should converge soon - instructions affected are instructions used for 32-bit pointer arithmetics, namely variants of add, sub and lea.

Review URL: http://codereview.chromium.org/3165059