meson: generate better arch defines for clang bpf compilation

The code assume that meson's cpu_family can be mapped directly to
'-D__<cpu_family>__'. This works in a surprising number of cases, but not for a
few architectures. PPC uses "powerpc", and RISC-V omits the trailing underscores.
ARM and RISC-V require a second define too.

Fixes #21900.

(I don't think this matters too much: we need *something* so that gnu/stubs.h
can be successfully included. But we don't actually call syscalls or depend too
much on the host environment, so things should be fine as long as we don't get
a compilation error.)

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