Modify toolchain.gypi to avoid adding -m64 flag on android.

The -m64 flag isn't supported on Android.  Add a similar gyp condition to avoid
setting the flag on Android in a similar manner to how the -m32 flag is avoided
on android.

BUG=354405
LOG=N
R=ulan@chromium.org

Review URL: https://codereview.chromium.org/212863007

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@20289 ce2b1a6d-e550-0410-aec6-3dcde31c8c00