Fix remaining CXX cross-compilation builds in Makefile.

r22509 fixed arm and mips (big-endian) cross-compilation builds.

It did not fix mipsel, mips64el, or arm64 builds, since the
toolchain prefix did not match the v8_target_arch.

For mips, the toolchain is typically named mips-linux-gnu-g++,
but it can compile for mipsel or mips64el via flags. (There can
also be mipsel-linux-gnu-g++ toolchains, and this patch will
work for mipsel build in that case).

For arm64, the toolchains seem to be named aarch64-linux-gnu-g++.

BUG=
R=jkummerow@chromium.org

Review URL: https://codereview.chromium.org/408133002

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@22531 ce2b1a6d-e550-0410-aec6-3dcde31c8c00