build: add x32 support to gyp build

This commit adds x32 support to the gyp build.  Configure with:

    $ ./gyp_uv.py -Dtarget_arch=x32
build: remove dtrace probes

The existing probes, all two of them, cause a great deal of pain for
people trying to build libuv on Linux because of SystemTap's dtrace(1)
utilitity not understanding the -xnolibs flag.

We could hack around that but it's easier to just remove the probes:
they are largely useless and unused while still needing a lot of
supporting infrastructure.  This commit removes 200 lines of code
and configuration.

Refs joyent/libuv#1478.