toolchain: make paranoid check of library/header paths unconditional

When we introduced support for the paranoid check of unsafe libraries
and headers path with commit 4ac8f78d (Add option for paranoid
unsafe path checking) back in 2014, we made it optional, as we expected
that would break quite a few packages.

Now, almost 8 years later, we only have three packages that explicitly
reference the option (dillo, gnuradio, and libtalloc), either in a patch
or in their .mk.

The option has been enabled by default since 2016, with 61c8854c
(toolchain: enable paranoid unsafe path check by default), and that has
not triggered many build failures in a while.

The minimal defconfig used by test-pkg has also had it enabled as of
b6c98b35 (minimal.config: add BR2_COMPILER_PARANOID_UNSAFE_PATH=y)
in 2017.

It is time to make that globally unconditional now.

There is still a remnant, in our binutils patches. As our toolchain may
get used outside of Buildroot, people may got the expectation that path
poisoning is only a warning, so we keep the current behaviour.

Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Romain Naour <romain.naour@gmail.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>