arm-none-eabi-gcc / 7-2017q4 precompiled binaries out of date

in the current 'package_index.json' the gcc/tools download is defined as:

```
"name": "arm-none-eabi-gcc",
"version": "7-2017q4",
"systems": [
{
"host": "arm-linux-gnueabihf",
"url": "http://downloads.arduino.cc/tools/gcc-arm-none-eabi-4.8.3-2014q1-arm.tar.bz2",
"archiveFileName": "gcc-arm-none-eabi-4.8.3-2014q1-arm.tar.bz2",
"checksum": "SHA-256:ebe96b34c4f434667cab0187b881ed585e7c7eb990fe6
b69be3c81ec7e11e845",
"size": "44423906"
},
```

however this points to an OLDER (4.8.3 2014q1) version of the compiler for arm-linux-gnueabihf, instead of the required version (7.2.1 2017q4). the windows/macosx/linux variables are correct; however the arm variants have not been precompiled with the right versions and are out of date.

this causes some samd projects to fail to compile when using the tools on a raspberry pi target.

The issue points to package_index.json; inspect the ARM tool entries and compare them with the 7-2017q4 metadata used by the Windows, macOS, and Linux entries. Verify the archive URLs, filenames, checksums, and sizes, then confirm a SAMD project builds with the ARM package on Raspberry Pi.

Written by the indexing model from the issue text.