SAMD21: Add build framework for DMA support.  Nothing there yet except for skeletal logic taken from SAM3/4.

STL32 F15x: stm32_stdclockconfig() was calling stm32_pw_setvos() which accessed PWR_CR via an inactive APB. From Juha Niskanen.

Add support for MK20DN--VLH5 and MK20DX---VLH5.  Needed for backward compatible support for Teensy-3.0

SAML21: Since SERCOM5 usese a different output channel, it will also need a different GCLK generator

Add support uClibc++ excpetions.  This involves additional handling for relative relation types, additional support for unwinding, as well as additional changes.  The culmination of a big effort fromo Leo Aloe3132