selftests: fix ARCH normalization to handle command-line argument

Several selftests Makefiles (e.g. prctl, breakpoints, etc) attempt to
normalize the ARCH variable by converting x86_64 and i.86 to

selftests: fix ARCH normalization to handle command-line argument

Several selftests Makefiles (e.g. prctl, breakpoints, etc) attempt to
normalize the ARCH variable by converting x86_64 and i.86 to x86.
However, it uses the conditional assignment operator '?='.

When ARCH is passed as a command-line argument (e.g., during an rpmbuild
process), the '?=' operator ignores the shell command and the sed
transformation. This leads to an incorrect ARCH value being used, which
causes build failures

# make -C tools/testing/selftests TARGETS=prctl ARCH=x86_64
make: Entering directory '/build/tools/testing/selftests'
make[1]: Entering directory '/build/tools/testing/selftests/prctl'
make[1]: *** No targets. Stop.
make[1]: Leaving directory '/build/tools/testing/selftests/prctl'
make: *** [Makefile:197: all] Error 2

Change the assignment to use 'override' and ':=' to ensure the
normalization logic is applied regardless of how the ARCH variable was
initially defined.

Link: https://lkml.kernel.org/r/20260309205145.572778-1-aleksey.oladko@virtuozzo.com
Signed-off-by: Aleksei Oladko <aleksey.oladko@virtuozzo.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Bala-Vignesh-Reddy <reddybalavignesh9979@gmail.com>
Cc: Chelsy Ratnawat <chelsyratnawat2001@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

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