Set arm_tune to empty string if mtune flag is not set by toolchain

Since the default arm_tune option applied by chromium is not
recognized by our gcc toolchain set arm_tune to empty string
to prevent chromium from setting a default. Our toolchains
set the arm version which already implies the corresponding
mtune option anyway.

Change-Id: I0291bd4f0e5706597d37f914e4eeefbd75397bbf
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>