build: don't compile with -B

Turn off the custom toolchain, we don't bundle ld.gold.  It's not that
useful anyway because it uses a fixed path.  For people that do use a
custom toolchain, there is the GCC_EXEC_PREFIX environment variable.

PR-URL: https://github.com/nodejs/node/pull/6393


Reviewed-By: Anna Henningsen <anna@addaleax.net>