build: add x32 support to gyp build

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

    $ ./gyp_uv.py -Dtarget_arch=x32

README.md

@@ -123,6 +123,8 @@ Run:
     $ ./gyp_uv.py -f make
     $ make -C out
 
+Run `./gyp_uv.py -f make -Dtarget_arch=x32` to build [x32][] binaries.
+
 ### OS X
 
 Run:

common.gypi

@@ -143,6 +143,10 @@
             'cflags': [ '-m32' ],
             'ldflags': [ '-m32' ],
           }],
+          [ 'target_arch=="x32"', {
+            'cflags': [ '-mx32' ],
+            'ldflags': [ '-mx32' ],
+          }],
           [ 'OS=="linux"', {
             'cflags': [ '-ansi' ],
           }],