[mips][mips64r6] b(ge|lt)zal are not available on MIPS32r6/MIPS64r6 and bal is a normal instruction

Summary:
b(ge|lt)zal have been removed in MIPS32r6/MIPS64r6. However, bal (an alias
for 'bgezal $zero, $offset') still remains with the same encoding it had
prior to MIPS32r6/MIPS64r6.

Updated the MipsNaCLELFStreamer, and MipsLongBranch to correctly handle the
MIPS32r6/MIPS64r6 BAL instruction in addition to the existing BAL_BR pseudo.

No changes were required to the CodeGen test that looks for BAL
(test/CodeGen/Mips/longbranch.ll) since the new instruction has the same
syntax.

Depends on D4113

Reviewers: jkolek, zoran.jovanovic, vmedic

Reviewed By: vmedic

Differential Revision: http://reviews.llvm.org/D4114