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Support for 64-bit aarch64 and mips64 are not great in gcc4.8, and x86_64
follows suite (to use gcc4.9 as the default) for the sake of consistency.

gcc-4.8 is removed from 64-bit.  As a result, clang with 64-bit target
now sets -gcc-toolchain to gcc-4.9

Change-Id: I47babe9bc34d041ea2a99b8839c8ca1688edb055