Support for custom NM/readelf binaries in your toolchain.

Background: nm and readelf are used by ninja.py to generate
.TOC files. These .TOC files are what determine when something
needs rebuilding.

ninja.py uses distinct commands for host vs target tools for most things
in the toolchain (cc, c++, ld,), via make_global_settings, but it fails to do so
for nm & readelf. This is not by design, but it has worked thus far by chance:
The default 'nm' and 'readelf' in most people's PATH are the system variants,
and the Linux versions of these tools happen to work.

However, the project I'm working on has engineers on Macs developing for
android. The system-supplied 'nm' and 'readelf' on Mac do NOT work for
e.g. Android arm binaries, which leads me to this fix. This fix allows for
specifying NM and READELF via make_global_settings, so we can point those
variables to the correct ones for the given toolchain/target.

BUG=
R=thakis@chromium.org

Review URL: https://codereview.chromium.org/164023009

git-svn-id: http://gyp.googlecode.com/svn/trunk@1971 78cadc50-ecff-11dd-a971-7dbc132099af