support/config-fragments/autobuild: update all pre-built Buildroot toolchains

support/config-fragments/autobuild: update all pre-built Buildroot toolchains

All toolchains have been rebuilt with Buildroot 2019.02-rc1.

Changes:

- Toolchains that were using no-longer maintained kernel headers
  versions have been changed to use a variety of newer kernel headers
  versions (4.4, 4.9 or 4.14).

- Since gcc 7.x is now the default in Buildroot, most toolchains that
  simply use the default gcc version use 7.x instead of 6.x.

- br-arm-cortex-a9-glibc uses gcc 8.x, binutils 2.31 and kernel
  headers 4.20

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>