Add getpid implementation for NaCl.

This change adds a getpid() implementation bases on the
IRT's GETPID interface.

In adding this I also updated the copy of nacl/irt.h
to match the latest version in the nacl repo.

BUG= http://code.google.com/p/nativeclient/issues/detail?id=2488
TEST= run_getpid_test
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/20204004
Add a PT_NOTE section to mark build_id.

Adding a PT_NOTE section to mark the location of the build_id note will allow
us to extract it from dl_iterate_phdrs at runtime.
This in turn will facilitate providing module information with minidumps.

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3607
TEST=None
R=mcgrathr@chromium.org, mseaborn@chromium.org

Review URL: https://codereview.chromium.org/21636003
Drop section rewriting and format fixes.

Turning off section rewriting in fixup_ldso that's not required to
satisfy sel_ldr anymore.

Formatting fixes left over from:
https://codereview.chromium.org/21636003/

BUG=https://code.google.com/p/nativeclient/issues/detail?id=3607
TEST=trybots
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/22527004
Reverting nacl_fixup_ldso.py sections change.

It turns out the loader does not actually allow all of the sections our glibc
toolchain emits (PT_DYNAMIC + rw in particular). We should fix this,
but for now making an ABI breaking change is undesirable.

BUG=None
TEST=trybots
R=mseaborn@chromium.org

Review URL: https://codereview.chromium.org/22756002