Arm userland not building with gcc-9 and later

[Jan Kiszka via Xenomai]

Hi all,

just wanted to debug the RTnet issues we now see in CI on arm and arm64.
I picked arm as first target, but that apparently starts to break with
gcc-9 or newer (tried 9.2 and 10.2):

make[5]: Entering directory '/xenomai/lib/cobalt/arch/arm'
  CC       libarch_la-features.lo
features.c: In function 'cobalt_arch_check_features':
features.c:82:1: error: r7 cannot be used in 'asm' here
   82 | }
      | ^

That seems to be related to passing the syscall number via r7 on ARM. Is
our ABI soon no longer compilable, or can we fix this?

Jan

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