RISC OS 5 clock is 1 hour slow when DST is active

[zedtoo-de]

Describe the bug
When running RISC OS 5 with Daylight Saving active, the clock display of !Alarm is one hour slow. During the winter months, the time is notionally correct.
This bug only occurs when RISC OS 5 is being emulated. RISC OS 3.7 always shows the correct time.

To Reproduce

1. Go to "Configure > Boot" and add !Alarm to the list of applications to run at startup.
2. Shut down and restart RPCEmu.
3. Notice that Alarm's clock display starts off showing the correct time, then after 5-10 seconds jumps backwards 1 hour.
4. Go to "Configure > Time and date" and manually set the clock to any time whatsoever. After a few seconds the time will once again jump to 1 hour behind the time displayed by macOS.

System information:

• Host machine: MacBook Pro 13-inch (2017 version)
• rpcemu-macos release: 0.9.1-pp3 (recompiler or interpreter)
• macOS version: Mojave 10.14.5
• Emulated processor: StrongARM Risc PC
• RISC OS version: 5.24 or 5.27(06-June-2019 build)

Additional context

• The bug only occurs if DST is active. If you happen to be in a locale where DST is not active, it may be necessary to adjust your macOS settings so that you are temporarily in a "DST active" locale in order to witness the bug.
• RPCEmu 0.8.14-caliston-dev1 also suffered from this bug.