Do you know in which version of binutils it was corrected? Because, I built gcc 4.9.4 with binutils 2.25.1, and I still reproduce the issue (see below, take a look at the ‘as’ call: the -mcpu flag is truncated). With gcc 4.8, 5.1, 5.2 and 5.3 (all with the same binutils), the issue doesn’t appear.
I see it in FSF binutils branch 2.26 and it was backported in Linaro branch: linaro_binutils-2_25-branch (which is available in our git repo: http://git.linaro.org/toolchain/binutils-gdb.git) for instance in Linaro 5.1 2015.08 the version is:
aarch64-linux-gnu/bin/as -v GNU assembler version 2.25.0 (aarch64-linux-gnu) using BFD version (GNU Binutils) 2.25.0 Linaro 2015.01-3-git