[MCJIT] Add a 'section_addr' builtin function to RuntimeDyldChecker.

The syntax of the new builtin is 'section_addr(<filename>, <section-name>)'
(similar to the stub_addr builtin, but without a symbol name). It returns the
base address of the given section in the given object file. This builtin makes
it possible to refer to the contents of sections that cannot contain symbols,
e.g. sections added by the linker itself, like __eh_frame.