Work around the stage0 sanity check

Add rv32e-targets to 'stage0 missing targets'. This prevents the error
"no such target exists in the target list".

src/bootstrap/src/core/sanity.rs

@@ -37,6 +37,9 @@ pub struct Finder {
 const STAGE0_MISSING_TARGETS: &[&str] = &[
     // just a dummy comment so the list doesn't get onelined
     "armv7-rtems-eabihf",
+    "riscv32e-unknown-none-elf",
+    "riscv32em-unknown-none-elf",
+    "riscv32emc-unknown-none-elf",
 ];
 
 /// Minimum version threshold for libstdc++ required when using prebuilt LLVM