vex-llvm: introduced a build configuration file

Introduced the file config.mk that currently must define
the variable LIBVEX_INSTALL_PATH that contains the path
to the valgrind installation directory.
vex-llvm: added VEX instrumentation callback stub

This callback is intended to implement VEX->LLVM translation.
vex-llvm: removed commented code
vex-llvm: added LLVM function toplevel wrap

This function is intended to hold LLVM IR generated from VEX IR.
vex-llvm: get the current LLVM context from the bound LLVM module
vex-llvm: added shortcuts for creating LLVM contants and integer types
vex-llvm: added initial support for visiting VEX IR