Modify the x86_64 assembler to no longer build object code

[MatthewKosloski]

Currently, the x86_64 assembler creates object code from a C source file (see here). I no longer think the assembler should have this responsibility. The assembler should just be responsible for linking and assembling.

The assembler should first check for the existence of the compiler's runtime object code file in the same directory as the compiler jar. If the assembler finds the object code file, then it can be linked with the compiler's outputted source file to produce an executable. If the assembler does not find the object code, then a message should be sent to the standard error stream and no resulting executable is created.

The reason for this change is that I no longer want to include the runtime source file and header file in future releases; just an object code file. This change will result in less release artifacts.