tests: adding the missing vega_x86_tag for the test_hello_se.py

The test_hello_se.py was unable to run due to missing the key for the
vega_x86_tag in isa_str_map.

Change-Id: I2c483c1c84f3968a64555edfec0e4b77dfbbd126
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/61630
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
Reviewed-by: Bobby Bruce <bbruce@ucdavis.edu>
Tested-by: kokoro <noreply+kokoro@google.com>

tests/gem5/hello_se/test_hello_se.py

@@ -52,6 +52,7 @@ isa_str_map = {
     constants.mips_tag: "mips",
     constants.riscv_tag: "riscv",
     constants.sparc_tag: "sparc",
+    constants.vega_x86_tag: "x86",
 }
 
 static_progs = {