Fix recognizing X64C in pkg/vm/tool/precompiler2.

TEST=dartfuzz
Change-Id: Ia1a716c703da2f9968d8d3f926d6be1aaca1ceaf
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/406182
Reviewed-by: Tess Strickland <sstrickl@google.com>
Commit-Queue: Tess Strickland <sstrickl@google.com>

pkg/vm/tool/shared_functions.sh

@@ -25,7 +25,8 @@ function host_arch() {
 
 function parse_target_arch() {
   case "$1" in
-  *X64)
+  *X64 | \
+  *X64C)
   echo "X64"
   ;;
   *ARM64 | \