[AVR] SPI atomicity fix avr (part 1/2 of #2376)

This pull request broke into 6 simpler commits the big patch in #2376.

/cc @xxxajk @PaulStoffregen 

Any reason this isn't in SPI.h? 

Also Should  be <pins_arduino.h> in order to speed up compiles.

Explanation:
Directories named by -I are searched before the standard system include directories.  If the directory dir is a standard system include directory, the option is ignored to ensure that the default search order for system directories and the special treatment of system headers are not defeated.

Headers whose names are enclosed in double-quotes ( "" ) shall be searched for first in the directory of the file with the #include line, then in directories named in -I options, and last in the standard system include directories. 
For headers whose names are enclosed in angle brackets ( "<>" ), the header shall be searched for only in directories named in -I options and then in the standard system include directories.