support/testing: fix systemd test by using a more recent gcc

support/testing: fix systemd test by using a more recent gcc

As reported on the mailing list [0], there is a build issue with systemd
245 when using gcc < 5.0, due to the following build issue:

    ../src/shared/gpt.c:7:9: error: initializer element is not constant
             { GPT_ROOT_X86,              "root-x86"              },

The pre-built external toolchain we have for armv5 (the default with
just BR2_arm=y) is a very old toolchain from CodeSourcery, which has a
gcc 4.8; we have no other pre-built toolchains for armv5, except by
using a custom one, like those from the Bootlin toolchain builder. But
using a custom toolchain is not nice, as we want our runtime test to
test nominal configurations.

So, switch the systemd tests to use a Cortex-A9, so that we can use the
ARM 2019.12 toolchain, and with VFP, so that it can boot in the qemu
vexpress machine we use for the test-cases.

Fixes:
    https://gitlab.com/buildroot.org/buildroot/-/jobs/498144403
    https://gitlab.com/buildroot.org/buildroot/-/jobs/498144405
    https://gitlab.com/buildroot.org/buildroot/-/jobs/498144406
    https://gitlab.com/buildroot.org/buildroot/-/jobs/498144408
    https://gitlab.com/buildroot.org/buildroot/-/jobs/498144410
    https://gitlab.com/buildroot.org/buildroot/-/jobs/498144412

[0] http://lists.busybox.net/pipermail/buildroot/2020-April/278931.html

Signed-off-by: Romain Naour <romain.naour@gmail.com>
[yann.morin.1998@free.fr:
  - just use cortex-a9_VFP, instead of using a bootlin toolchain
  - adapt the commit log accordingly
]
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>