scons: Not compiling with systemc when host is RISCV

Compiling gem5 with systemc enabled results in the errors
described in https://gem5.atlassian.net/browse/GEM5-1027.

This change tells scons not to compile gem5 with systemc
when the host ISA is RISC-V.

JIRA: https://gem5.atlassian.net/browse/GEM5-1027

Change-Id: I04e1bc722c9d702472152c9116d89938e8408047
Signed-off-by: Hoa Nguyen <hoanguyen@ucdavis.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/47660
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>
1 file changed