chomeapps/ssh_client/build.sh: get rid of pnacl-vs-nacl

The link that PNACL_SDK is set to
($SDK_ROOT/trunk.169649/naclsdk_linux.tar.bz2) is broken.  Let's not
rely on the trunk SDK anymore, and use the stable SDK which is readily
available on the native-client page.  It makes no differentiation
between NaCl and PNaCl, as the latter is default now anyway.  So,
update the link and remove all the NACL-specific code in the script.

Also introduce a new variable GLIBC_TOOLCHAIN, which determines
whether to build using the glibc toolchain; when set to 0 (default),
use the newlib toolchain.  The code for the building with the GLIBC
toolchain is taken from the NACL-specific codepath.

Caveat: nacl-openssh-5.9p1.sh is still broken.  We plan to fix this in
a subsequent patch.

BUG=None
TEST=build.sh works after applying the patch.

Change-Id: Iff7e722b8d94baa31b1bcf80d4ca7e918a8f00e4