[AIX][test/Index] Set/propagate AIXTHREAD_STK for AIX

[hubert.reinterpretcast]

Summary

Some tests perform deep recursion, which requires a larger pthread stack size than the relatively low default of 192 KiB for 64-bit processes on AIX. The AIXTHREAD_STK environment variable provides a non-intrusive way to request a larger pthread stack size for the tests. The required pthread stack size depends on the build configuration.

A 4 MiB default is generous compared to the 512 KiB of macOS; however, it is known that some compilers on AIX produce code that uses comparatively more stack space.

[daltenty]

LGTM

test/Index/lit.local.cfg

+import platform
+
+# Some tests perform deep recursion, which requires a larger pthread stack size
+# than the relatively low default of 192 KiB for 64-bit processes on AIX. The
+# `AIXTHREAD_STK` environment variable provides a non-intrusive way to request
+# a larger pthread stack size for the tests. Various applications and runtime
+# libraries on AIX use a default pthread stack size of 4 MiB, so we will use
+# that as a default value here.
+if 'AIXTHREAD_STK' in os.environ:
+    config.environment['AIXTHREAD_STK'] = os.environ['AIXTHREAD_STK']
+elif platform.system() == 'AIX':
+    config.environment['AIXTHREAD_STK'] = '4194304'