support/testing: test_openrc: switch to ARM arm toolchain and builtin kernel

support/testing: test_openrc: switch to ARM arm toolchain and builtin kernel

The kernel 5.5.7 curently used by the test doesn't build with host
gcc >= 10 due the gcc default -fno-common. See GCC 10 porting guide [1].

/usr/bin/ld: scripts/dtc/dtc-parser.tab.o:(.bss+0x20): multiple definition of `yylloc'; scripts/dtc/dtc-lexer.lex.o:(.bss+0x0): first defined here

But we can't just update this test to the next linux kernel LTS 5.10.y since
the minimum gcc version has been updated to gcc 4.9 since 5.8 kernel [2]
and the Sourcery CodeBench ARM 2014.05 is used (gcc 4.8 based).

Enable arm cortex A9 and VFP support to switch to the ARM arm prebuilt
toolchain (the Bootlin toolchain could be used).
While at it use the prebuilt buildin kernel for the vexpress target
recently updated to 5.10.7.

Fixes:
https://gitlab.com/kubu93/buildroot/-/jobs/1564202094

[1] https://gcc.gnu.org/gcc-10/porting_to.html
[2] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=6ec4476ac82512f09c94aff5972654b70f3772b2
[3] 3cf2782906d5cc05a3966c25518d2ce01f11afc8

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