[GCC] Add support for INCLUDE statements

[praqma-thi]

The GCC parser (and likely the others) lacks support for INCLUDE statements, causing the parser to fail or build an incomplete map when contents are spread over multiple files.

Example can be found in issue #55, where the parser failed to find the SECTIONS block as this was located in an INCLUDEd file.

This'll likely result in pre-parsing the file for INCLUDE statements, recursively checking its INLCUDEs, etc. to build up a collection of files to include in parsing.