ci: fix ABI reference generation

ci: fix ABI reference generation

The machine=generic is not understood by older version of dpdk.
It is directly passed to gcc as -march=generic.

Since DPDK requires SSE 4.2, this results in an error when configuring
v21.02 sources for generating the reference ABI.

From GHA [1] logs:

"""
Compiler for C supports arguments -Wundef: YES
Compiler for C supports arguments -Wwrite-strings: YES
Compiler for C supports arguments -Wno-address-of-packed-member
-Waddress-of-packed-member: NO
Compiler for C supports arguments -Wno-packed-not-aligned
-Wpacked-not-aligned: NO
Compiler for C supports arguments -Wno-missing-field-initializers
-Wmissing-field-initializers: YES

config/x86/meson.build:14:6: ERROR:  Could not get define '__SSE4_2__'

A full log can be found at
/home/runner/work/dpdk/dpdk-v21.02/build/meson-logs/meson-log.txt
Error: Process completed with exit code 1.
"""

1: https://github.com/ovsrobot/dpdk/runs/2355005702

Stick to a compatible configuration passing -Dmachine=default.

Note: the breakage was not seen earlier this week as I guess the CI
workers are using a cached ABI reference for v20.11.

Fixes: 5b3a6ca6fd28 ("build: alias default build as generic")

Signed-off-by: David Marchand <david.marchand@redhat.com>
Reviewed-by: Juraj Linkeš <juraj.linkes@pantheon.tech>