Fix building mono with icc

When compiling mono with icc (ICC) 14.0.0 20130728 the build fails on

  CC     libmonoruntimesgen_la-sgen-stw.lo

with the following error:

Assembler messages:
Error: suffix or operands invalid for `lea'

Apparently icc (or the assembler) dislikes the rip-based LEA
operation. This can be worked around by manually assembling the
instruction and replacing it with the corresponding byte sequence.

This change is released under the MIT license.