Fixed gdb support and added some ElfFile functions

Fixed gdb support and added some ElfFile functions

Fixed gdb support so that it would continue working even when debug
symbols or other sections are included in the elf file. Also made it
actually read parts of the DWARF information so it should work even if
there are minor changes to how and where DWARF information is written
out.

Added a dwarf.h file with the dwarf constants.

Added a FindSectionByName function, a FindDynamicSymbol function, and
the ability to specify the mmap protection and flags directly if we are
mapping in the whole file.

Modified elf_writer_quick.cc to use the dwarf constants from dwarf.h.

Change-Id: I09e15c425fab252b331a2e4719863552e8b6b137