android envsetup: Stop honoring --target-arch parameter.

Most people don't use this parameter and get arm binaries. This is
still the default, so this change shouldn't affect most people.

Folks should instead pass -Dtarget_arch to gyp. (Or, soon, envsetup will stop
clobbering GYP_DEFINES, then you can just add target_arch to your GYP_DEFINES.)
Note that in gyp land, 'mips' is called 'mipsel' and 'x86' is called 'ia32'.
'arm' stays 'arm'.

So for example, instead of running

  . build/android/envsetup.sh --target-arch=mips
  android_gyp

you'd run

  . build/android/envsetup.sh
  android_gyp -Dtarget_arch=mipsel

I updated the bots I was able to find to pass the -D flag in addition to
--target-arch. After this CL here is in, I'll update the bots to stop passing
--target-arch, and then I'll make --target-arch a hard error in this script for
a while, to make sure nobody still uses it.

BUG=330631,34476
R=torne@chromium.org
TBR=yfriedman

Review URL: https://codereview.chromium.org/171903002

git-svn-id: svn://svn.chromium.org/chrome/trunk/src@252034 0039d316-1c4b-4281-b951-d872f2087c98
SVN changes up to revision 252034