[MC] Enable eip-relative addressing on x86-64 for X32 ABI

Summary:
Enables eip-based addressing, e.g.,

lea    constant(%eip), %rax
lea    constant(%eip), %eax

in MC, (used for the x32 ABI). EIP-base addressing is also valid in x86_64,
it is left enabled for that architecture as well.

Patch by João Porto

Differential Revision: http://reviews.llvm.org/D16581

llvm-svn: 259528