linker: improve loadable segment protection.

Use the functions in linker_phdr.c to load the PT_LOAD segments
in memory, and toggle their mapping's writable protection bit
as needed. In particular:

  - when loading a library, load the segments then unprotected
    them to allow relocations to work.

  - when relocating the linker of the executable, unprotect
    the segments loaded by the kernel to make relocations work

  - after all relocations are done, re-protect the segments,
    and apply GNU RELRO protection if needed.

  - just before calling the destructors, undo the GNU RELRO

Change-Id: I50e709f03958204b8d6140c0f51ebe24fe089a1b
2 files changed