i#4953 Ubuntu20: Fix linux.syscall_pwait hang. (#5132)

Fixes hang at the syscall ppoll_time64 by handling it same as ppoll.
The former is the year-2038-safe variant of the latter, that allows
a wider field for timestamp, as the year 2038 will see a unix
timestamp greater than 2^31.

Adds new syscalls up to Linux kernel 5.10.46.

Adds a warning for new syscalls other than ppoll_time64 that are
not handled yet and have a similarly named variant that has
special handling. E.g. SYS_clone has special handling, so added
a warning for SYS_clone3.

Also upgrades Ubuntu version for the x86-32 bit GA CI to 20.04.

Issue: #5131, #4953

.github/workflows/ci-x86.yml

@@ -55,8 +55,7 @@ defaults:
 jobs:
   # 32-bit Linux build with gcc and run tests:
   x86-32:
-    # TODO i#4953: Upgrade to ubuntu-20.04, same as other CI jobs.
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-20.04
 
     # When checking out the repository that triggered a workflow, actions/checkout@v2
     # defaults to the reference or SHA for that event. See documentation for ref at