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>

src/systemc/SConsopts

@@ -28,6 +28,14 @@ Import('*')
 from gem5_scons import warning
 
 def use_systemc_check(env, warn=False):
+    import platform
+    host_isa = platform.machine()
+    # Disallow compiling systemc if the host ISA is RISC-V.
+    # The error is described here: https://gem5.atlassian.net/browse/GEM5-1027
+    if host_isa.startswith("riscv"):
+        if warn:
+            warning('Warning: Systemc may not work on RISC-V.')
+        return False
     if env['PLATFORM'] == 'darwin':
         if warn:
             warning('Warning: Systemc may not work on Mac OS.')