[llvm-jitlink] Add -alias option, shorten "-define-abs" option to "-abs".

The -alias option can be used to define aliases within a JITDylib. The
immediate motivation is to simplify testing of ORC runtime functions using
existing testcases (e.g. by aliasing dlfcn functions to their ORC-runtime
counterparts, like -alias dlopen=__orc_rt_macho_dlopen). The option is likely
to be useful for testing in general.

The -define-abs option is shortened to -abs for consistency with -alias.