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.
Fixed bug where divide by zero bugs would only be detected once in a program
even if there were many divide by zero bugs.

The fix basically inlines all function calls to klee_div_zero_check()
so that each call to klee_report_error() is a unique instruction
for each instrumentation of a divide operation.

It also seems that inlining the call "magically" fixed the debug information
(file and line number) of the instruction so that the debug information on the
inlined instructions matches that of the instrumented division instruction.

Note that the command line option -emit-all-errors could be used to
workaround the bug fixed in this commit.
Implemented runtime check for overshift (controllable with --check-overshift
command line argument).

Overshift is where a Shl, AShr or LShr has a shift width greater
than the bit width of the first operand. This is undefined behaviour
in LLVM so we report this as an error.