build: Updates for AIX npm support - part 1

build: Updates for AIX npm support - part 1

This PR is the first step enabling support for native modules
for AIX.  The main issue is that unlike linux where all
symbols within the Node executable are available to the shared
library for a native module (npm), on AIX the symbols must
be explicitly exported.  In addition, when the shared library is
built it must be linked using a list of the available symbols.

This patch covers the changes need to:

 1) Export the symbols when building the node executable
 2) Generate the file listing the symbols that can be used when
    building the shared library.

For AIX, it breaks the build process into 2 steps.  The first builds
a static library and then generates a node.exp file which contains
the symbols from that library.  The second builds the node executable
and uses the node.exp file to specify which symbols should be
exported.  In addition, it save the node.exp file so that it can
later be used in the creation of the shared library when building
a native module.

The following additional steps will be required in dependent projects
to fully enable AIX for native modules and are being worked
separately:

 - Updates to node-gyp to use node.exp when creating the
   shared library for a native module
 - Fixes to gyp related to copying files as covered in
      https://codereview.chromium.org/1368133002/patch/1/10001
 - Pulling in updated gyp versions to Node and node-gyp
 - Pulling latest libuv

These changes were done to minimize the change to other platforms
by working within the existing structure to add the 2 step process
for AIX without changing the process for other platforms.

PR-URL: https://github.com/nodejs/node/pull/3114
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>