[builders] Add simriscv32 builders.

This is follow-up to a6547c5

Change-Id: I7d2c6584eb1cb5d694eda2c79ad323ffb7a7dffc
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/393520
Reviewed-by: Ryan Macnak <[email protected]>
Reviewed-by: Siva Annamalai <[email protected]>
Commit-Queue: Alexander Aprelev <[email protected]>

Author: aam

tools/bots/test_matrix.json

@@ -1056,6 +1056,7 @@
     },
     {
       "builders": [
+        "vm-linux-debug-simriscv32",
         "vm-linux-debug-simriscv64",
         "vm-linux-debug-x64c",
         "vm-linux-release-simarm",
@@ -1152,6 +1153,7 @@
     },
     {
       "builders": [
+        "vm-aot-linux-debug-simriscv32",
         "vm-aot-linux-debug-simriscv64",
         "vm-aot-linux-debug-x64c",
         "vm-aot-linux-product-x64",