Add ATtiny1634 core support

[GlassOnTin]

Summary

Adds support for ATtiny1634 microcontroller.

ATtiny1634 specifications:

• 16KB Flash, 1KB SRAM, 256B EEPROM
• 3 GPIO ports (A: 8 pins, B: 4 pins, C: 6 pins)
• Timer0: 8-bit with PWM on OC0A (PC0), OC0B (PA5)
• Timer1: 16-bit with PWM on OC1A (PB3), OC1B (PA6)
• 12-channel 10-bit ADC with temperature sensor
• Watchdog timer, PCINT on all 3 ports

Implementation notes:

• ATtiny1634 has no WDCE bit, so watchdog is declared manually without using the AVR_WATCHDOG_DECLARE macro
• All WGM modes for both timers are implemented including Fast PWM with OCRnA as TOP (mode 7 for Timer0, mode 15 for Timer1)
• External clock pins for timers are supported

Testing:

• Compiles successfully with simavr build system
• Core is auto-detected and added to sim_core_config.h

Use case:

This MCU is used in flashlight firmware (Anduril) and having simavr support enables firmware development and testing without physical hardware.