support/config-fragments/autobuild: set gcc version for RISC-V toolchains

support/config-fragments/autobuild: set gcc version for RISC-V toolchains

Prior to b3ba26150d49dbc6b06ad402c5f6fbd398a71451
("toolchain/toolchain-external/toolchain-external-custom: be more
flexible on gcc version"), the default gcc version selected by
Buildroot for custom external toolchain was affected by the
BR2_ARCH_NEEDS_GCC_AT_LEAST_xyz definitions.

Since BR2_riscv selects BR2_ARCH_NEEDS_GCC_AT_LEAST_7, gcc 7.x was the
default gcc version assumed to be used in a custom RISC-V external
toolchain, so our config snippets for RISC-V toolchains were correct.

With b3ba26150d49dbc6b06ad402c5f6fbd398a71451 applied, the default gcc
version assumed for custom external toolchains is the latest one
(currently gcc 9.x), while our RISC-V toolchains use gcc 7.x. So we
now need to explicitly give the gcc version used by our RISC-V
toolchains, otherwise the build fails with:

  Incorrect selection of gcc version: expected 9.x, got 7.4.0

Fixes:

  http://autobuild.buildroot.net/results/b872befe1adec2633b9cbcc49bc0eb7619f606c2/

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>