x32. Fixes a bug in x32 exception handling.

This patch updates the X86 lowering so that the Exception Pointer and Selector
are 64-bit wide only if Subtarget.isTarget64BitLP64.

Patch by João Porto

Reviewers: dschuff, rnk
Differential Revision: http://reviews.llvm.org/D12111

llvm-svn: 245454