build: Allow passing CFLAGS to guest compile on the Make command line.

Signed-off-by: Diego Biurrun <diego@biurrun.de>
build: Compile guest tools for native x86 architecture variant.

Forcing 32 bits is a remnant from the times when S2E did not support
64 bits environments natively.

Signed-off-by: Diego Biurrun <diego@biurrun.de>