[EABI] Add Clang support for -meabi flag

The -meabi flag to control LLVM EABI version.

Without '-meabi' or with '-meabi default' imply LLVM triple default.
With '-meabi gnu' sets EABI GNU.
With '-meabi 4' or '-meabi 5' set EABI version 4 and 5 respectively.

A similar patch was introduced in LLVM.

Patch by Vinicius Tinti.

llvm-svn: 252463