macos: Improve macOS 10.12 support. bz#365327.

macos: Improve macOS 10.12 support. bz#365327.

The new macOS 10.12 way of loading dylib (placing them at the end of the currently
loaded segments) requires that Valgrind needs to know where the last segment was loaded.

A new structure (load_info_t) has been created to store all this information and easily
carry it around.

Changes:
- dyld text address is relative instead of absolute (macOS 10.12)
- Handle dylinker's offset (macOS 10.12)
- Handle the executable requesting a non-default stack address (macOS 10.12)
- Refactor to use load_info_t structure (all macOS)

Patch from Louis Brunner.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@16318