Fix #223

See https://github.com/arduino/Arduino/issues/4525

PWM frequency 732 Hz

The RXC interrupt was being disabled when using bit to disable the DRE
interrupt.

These functions were changed from private to protected but the comments were not updated at that time.

In conjunction with equivalent pull requests to Arduino AVR Boards and Arduino SAM Boards, solves #6146.

This enables remote wakeup on CDC driver, so we can safely call standby() and maintain the Serial prints on resume.

Fix #216

CMSIS-Atmel 4.5 changed the prescaler defines to match the bit mask, not the actual prescaler value, thus `SYSCTRL_OSC8M_PRESC_1_Val`  now means "divide by 2"

Fixes https://github.com/arduino/ArduinoCore-samd/issues/214

Via 2nd stage boot code that checks SD card for UPDATE.bin

    * cores/arduino/wiring.c:

Use the information retrieved from the linker script in use to simply call a reset without any erase

The exported variable is used by banzai() to avoid erasing any bootloader part

Fixes some compile errors and warnings

This pachs allows C++ global constructors to run after hardware
initialization. This helps some libraries that setups hardware
in class constructor to work properly.

See also #169

Fix #172