[libFuzzer] Instrument bcmp

If we define memcmp in an archive, bcmp should be defined as well (many libc
define bcmp/memcmp in one object file).  Otherwise if the application calls bcmp
or strcmp which gets optimized to bcmp (SimplifyLibCalls), the undefined
reference may pull in an optimized bcmp/strcmp implementation (libc replacement)
later on the linker command line.  If both libFuzzer's memcmp and the optimized
memcmp are strong => there will be a multiple definition error.