envsetup.sh: support setting arm-eabi toolchain as clang 3.4

Summary:
Our wrapper and toolchain now support building for arm-eabi
even with kernels. Because of this, TARGET_ARM_EABI_VERSION
may now be specified as clang or clang-3.4

Test Plan: verify

Reviewers: randall.rushing, synergy

Reviewed By: synergy

Differential Revision: http://review.codefirex.com/D590

Change-Id: I57555bc282b5aaf9c5c39b7296b9cdda984f1232