Add options to allow for specifying an option string when spawning gdb and for executing

command(s) right after starting up gdb.  Update the README file to show an example of
using these to pass '-arch armv7' to gdb and to execute gdb command to set shared library
path substitutions before loading iOS4.3 sdk's /usr/lib/libSystem.B.dylib and disassembling
the 'printf' function.

llvm-svn: 128040