Fix an x86 assembler stack unwind calculation for non-volatile registers.

This change has the practical effect of fixing some backtrace
scenarios that would fail with inferiors running on the Android Art
host-side JVM under Linux x86_64 on Ubuntu 14.04.

See this lldb-commits thread for more details:
http://lists.cs.uiuc.edu/pipermail/lldb-commits/Week-of-Mon-20140721/011988.html

Change by Tong Shen.
Reviewed by Jason Molenda.

Tested:
Ubuntu 14.04 x86_64, clang-3.5-built lldb.
MacOSX 10.10 Preview 4, Xcode 6 Beta 4-built lldb.