introduce vts_kernel_net_test_32 helper target

(tested on git_main with aosp_cf_x86_64_phone-trunk_staging-userdebug)

Without this you only get 32-bit testing if you run 'atest -a' or full VTS:

$ atest vts_kernel_net_tests
x86_64 vts_kernel_net_tests: Passed: 650, Failed: 0, Ignored: 3, Assumption Failed: 0

vs

$ atest -a vts_kernel_net_tests
x86_64 vts_kernel_net_tests: Passed: 650, Failed: 0, Ignored: 3, Assumption Failed: 0
x86 vts_kernel_net_tests: Passed: 198, Failed: 0, Ignored: 455, Assumption Failed: 0

With this even a non '-a' atest run will test both 32-bit and 64-bit:

$ atest vts_kernel_net_tests{,_32}
x86_64 vts_kernel_net_tests: Passed: 650, Failed: 0, Ignored: 3, Assumption Failed: 0
x86 vts_kernel_net_tests_32: Passed: 198, Failed: 0, Ignored: 455, Assumption Failed: 0

(note that the latter in spite of being tagged x86_64 is really x86 as can be seen by ignored count)

While a run with '-a' unfortunately duplicates the 32-bit tests.

$ atest -a vts_kernel_net_tests{,_32}
x86_64 vts_kernel_net_tests: Passed: 650, Failed: 0, Ignored: 3, Assumption Failed: 0
x86 vts_kernel_net_tests: Passed: 198, Failed: 0, Ignored: 455, Assumption Failed: 0
x86 vts_kernel_net_tests_32: Passed: 198, Failed: 0, Ignored: 455, Assumption Failed: 0

The intent of this change is for us to notice 32-bit vts test failures earlier,
rather than waiting on vendors & oems to report them.

It doesn't look like anyone runs 'atest -a' in automation...

/google/bin/releases/jetski-devs/tools/cli --conversation=2cbf222c-8d3e-44ff-a00e-a26ae0da5c36

Bug: 532739201
Test: TreeHugger, atest -a vts_kernel_net_tests{,_32}
Flag: EXEMPT BUG_FIX
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I00c0a38405d514d706a43faedaae70a95e73cb96