Our infinite_recurse() test to verify sane stack-overrun behavior
is affected by a bug of the Linux kernel on PPC64: it will get SIGSEGV
if it receives a signal when the stack depth is (a) over 1MB and
(b) within a few kB of filling the current physical stack allocation.
See https://bugzilla.kernel.org/show_bug.cgi?id=205183.

Since this test is a bit time-consuming and we run it in parallel with
test scripts that do a lot of DDL, it can be expected to get an sinval
catchup interrupt at some point, leading to failure if the timing is
wrong.  This has caused more than 100 buildfarm failures over the
past year or so.

While a fix exists for the kernel bug, it might be years before that
propagates into all production kernels, particularly in some of the
older distros we have in the buildfarm.  For now, let's just back off
and not run this test on Linux PPC64; that loses nothing in test
coverage so far as our own code is concerned.

To do that, split this test into a new script infinite_recurse.sql
and skip the test when the platform name is powerpc64...-linux-gnu.

Back-patch to v12.  Branches before that have not been seen to get
this failure.  No doubt that's because the "errors" test was not
run in parallel with other tests before commit 798070ec, greatly
reducing the odds of an sinval catchup being necessary.

I also back-patched 3c855354 into v12, just so the new regression
script would look the same in all branches having it.

Discussion: https://postgr.es/m/3479046.1602607848@sss.pgh.pa.us
Discussion: https://postgr.es/m/20190723162703.GM22387%40telsasoft.com