support/config-fragments/autobuild: use external toolchains in RISC-V configs

support/config-fragments/autobuild: use external toolchains in RISC-V configs

This commit replaces the two RISC-V configurations used for the
autobuilders to use pre-built external toolchains rather than internal
toolchains. This saves quite a bit of build time in the autobuilders,
and also allows people to reproduce build issues in a much more
efficient way, since rebuilding the toolchain is not needed.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Cc: Mark Corbin <mark.corbin@embecosm.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>