Show "Global variables use ..." during build of SAM boards

[ollie1400]

This PR is in answer to https://github.com/arduino/Arduino/issues/5245

Adds a few config lines following suggestions in that issue. If I make these changes locally (in my arduino installation, it seems to get the desired result):

I have set the maximum size based on the size of the ram section (ram (rwx) : ORIGIN = 0x20070000, LENGTH = 0x00018000 /* sram, 96K */).

I am not very familiar with all of the machinery in the Arduino app, but I trust that this fix is correct because looking in the map file, the BSS section size matches the number in the Arduino IDE output.

[CLAassistant]

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