Modified the buildmode of bitcode libraries.

The Default is Release+Asserts but if you are building KLEE with debug symbols (for example
"Release+Debug+Asserts" or "Debug+Asserts") then this breaks because KLEE will
look for the bitcode libraries in the wrong place because the
RUNTIME_CONFIGURATION macro is not defined to be what KLEE actually builds as.

This has been tweaked so that when we build the bitcode libraries the Makefile
variable "DEBUG_SYMBOLS" is correctly overridden.