Add linux-gcc-6.3 modular builder #3571

Job	Run time
raspi-2 / initialize	38s
raspi-2-skia / initialize	45s
raspi-2 / docker-build-image	27s
raspi-2 / docker-unittest-image	0s
raspi-2-skia / docker-build-image	27s
raspi-2-skia / docker-unittest-image	0s
raspi-2 / raspi_devel	9m 10s
raspi-2 / raspi_debug	10m 9s
raspi-2 / raspi_qa	8m 20s
raspi-2 / raspi_gold	7m 39s
raspi-2 / sbversion-13_devel	9m 15s
raspi-2 / sbversion-13_debug	9m 32s
raspi-2 / sbversion-13_qa	7m 48s
raspi-2 / sbversion-13_gold	7m 42s
raspi-2 / sbversion-14_devel	9m 41s
raspi-2 / sbversion-14_debug	10m 43s
raspi-2 / sbversion-14_qa	7m 58s
raspi-2 / sbversion-14_gold	7m 38s
raspi-2 / sbversion-15_devel	8m 52s
raspi-2 / sbversion-15_debug	11m 32s
raspi-2 / sbversion-15_qa	7m 32s
raspi-2 / sbversion-15_gold	8m 6s
raspi-2-skia / raspi-2-skia_devel	9m 3s
raspi-2-skia / raspi-2-skia_debug	10m 40s
raspi-2-skia / raspi-2-skia_qa	8m 56s
raspi-2-skia / raspi-2-skia_gold	7m 48s
raspi-2-skia / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
raspi-2-skia / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2 / ${{ matrix.name }}_${{ matrix.shard }}_test	0s
raspi-2 / ${{ matrix.name }}_on_device_${{ matrix.shard }}	0s
	3h 0m 21s

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