Scons: Move the creation of base_env into a function
Scons: Move the creation of base_env into a function

As with the earlier refactoring, this will make it easier to
instantiate envs twice, for x86-32 and x86-64.  It is also cleaner to
move code out of the Python top level.

The help text and COVERAGE_TARGETS can move to pre_base_env since they
are not trusted-env-specific.

BUG=http://code.google.com/p/nativeclient/issues/detail?id=1988
TEST=trybots

Review URL: http://codereview.chromium.org/7385003

git-svn-id: 721b910a23eff8a86f00c8fd261a7587cddf18f8@6073 fcba33aa-ac0c-11dd-b9e7-8d5594d729c2

sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,nacl-webkit/native_client,sbc100/native_client,sbc100/native_client