website: added RISCV to the valid ISAs list

Change-Id: Ief7746d2bf65cd119dd223b96027d825cc2cb481
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5-website/+/52343
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Bobby R. Bruce <bbruce@ucdavis.edu>
Tested-by: Bobby R. Bruce <bbruce@ucdavis.edu>
diff --git a/_pages/documentation/general_docs/building/index.md b/_pages/documentation/general_docs/building/index.md
index d4cebda..ab5a8ba 100644
--- a/_pages/documentation/general_docs/building/index.md
+++ b/_pages/documentation/general_docs/building/index.md
@@ -153,6 +153,7 @@
 * NULL
 * MIPS
 * POWER
+* RISCV
 * SPARC
 * X86