32-bit signal tests failing on Ubuntu 20.04

Xref the issue for upgrading Ubuntu version to 20.04 for our Github CI #4953. Creating a separate one for the following problem:

Many tests seem to be failing due to an abrupt end of logs:

```
Test #42: code_api|linux.signal0000 ........................................***Failed Required regular expression not found. Regex=[^Sending SIGUSR2
2021-06-30T16:31:59.0520704Z Sending SIGUSR1
2021-06-30T16:31:59.0521168Z in signal handler
2021-06-30T16:31:59.0521616Z Got SIGUSR1
2021-06-30T16:31:59.0522067Z Sending SIGRTMAX
2021-06-30T16:31:59.0522530Z in signal handler
2021-06-30T16:31:59.0522978Z Got SIGRTMAX
2021-06-30T16:31:59.0523454Z Generating SIGSEGV
2021-06-30T16:31:59.0523928Z in signal handler
2021-06-30T16:31:59.0524373Z Got SIGSEGV
2021-06-30T16:31:59.0524760Z 250006\.749760
2021-06-30T16:31:59.0525108Z $
2021-06-30T16:31:59.0525465Z ] 0.77 sec
2021-06-30T16:31:59.0525884Z Sending SIGUSR2
2021-06-30T16:31:59.0526334Z Sending SIGUSR1
2021-06-30T16:31:59.0526796Z in signal handler
2021-06-30T16:31:59.0527239Z Got SIGUSR1
2021-06-30T16:31:59.0527508Z
```
https://github.com/DynamoRIO/dynamorio/pull/4981/checks?check_run_id=2954404259

All of the following tests seem to have a similar issue:

```
code_api|linux.signalxxxx
code_api|linux.sigplainxxx

code_api|linux.syscall_pwait
code_api|linux.sigcontext

code_api|linux.signal_race
code_api|linux.alarm
code_api|linux.bad-signal-stack
Code_api|linux.sigsuspend
Code_api|linux.signest
Code_api|client.events
Code_api|client.events_cpp
Code_api|client.cleancallsig
Code_api|client.signal
code_api|api.static_signal
Code_api,opt_memory|client.events
Code_api,opt_speed|client.events
Code_api,thread_private|client.events
Code_api,disable_traces|client.events
Code_api,thread_private,disable_traces|client.events
```