[AVR] Add support for the -mdouble=x flag

This flag is used by avr-gcc (starting with v10) to set the width of the
double type. The double type is by default interpreted as a 32-bit
floating point number in avr-gcc instead of a 64-bit floating point
number as is common on other architectures. Starting with GCC 10, a new
option has been added to control this behavior:
https://gcc.gnu.org/wiki/avr-gcc#Deviations_from_the_Standard

This commit keeps the default double at 32 bits but adds support for the
-mdouble flag (-mdouble=32 and -mdouble=64) to control this behavior.

Differential Revision: https://reviews.llvm.org/D76181