GCC version 6 -> 8

Author: kjbracey

docs/tools/tools_intro.md

@@ -26,7 +26,7 @@ Mbed OS 5 can be built with various toolchains. The currently supported versions
   - A paid version is available as [Arm Compiler 6.11 Professional](https://developer.arm.com/tools-and-software/embedded/arm-compiler/downloads/version-6).
   - A paid version is also included in [Keil MDK 5.27](http://www2.keil.com/mdk5/).
 - [Arm Compiler 5.06 update 6 (to be deprecated in the future)](https://developer.arm.com/tools-and-software/embedded/arm-compiler/arm-compiler-5/downloads).
-- [GNU Arm Embedded version  6 (6-2017-q1-update)](https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads).
+- [GNU Arm Embedded version 8 (8-2018-q4-major)](https://developer.arm.com/tools-and-software/open-source-software/developer-tools/gnu-toolchain/gnu-rm/downloads).
 - [IAR Embedded Workbench 8.32.1](https://iar.com/mbed).
 
 #### Arm Mbed Studio