support/testing: switch to prebuilt toolchain, core2duo to Nehalem

support/testing: switch to prebuilt toolchain, core2duo to Nehalem

To avoid spending some time to build the x86_64 toolchain (~20min),
switch to corei7 cpu (Nahalem) and use the prebuilt Bootlin toolchain.

We have to use the "stable" Bootlin toolchain to use the same kernel version
for the toolchain kernel headers and the running kernel.

With the "bleeding-edge" toolchain we have the "kernel too old" issue
(running kernel 4.19 vs kernel headers 5.4)

Runtime tested locally.

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Cc: Peter Korsgaard <peter@korsgaard.com>
Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>