revert to legacy (8-2018-q4-major) compiler

Moving to the 7-2017q4 version used by the Arduino Nano 33 BLE caused some nasty bugs (see #112) so for now we are going back. Will need to perform further testing regarding upcoming TensorFlow examples

Author: Owen L - SFE

platform.txt

@@ -63,7 +63,7 @@ defines.variant={build.defs}
 
 
 ## Compiler and Toolchain
-compiler.path={runtime.tools.arm-none-eabi-gcc-7-2017q4.path}/bin
+compiler.path={runtime.tools.arm-none-eabi-gcc-8-2018-q4-major.path}/bin
 compiler.cmd.cpp=arm-none-eabi-g++
 compiler.cmd.c=arm-none-eabi-gcc
 compiler.cmd.S=arm-none-eabi-gcc