misc: string.join has been removed in python3

In general string methods are deprecated in favour of str ones

Change-Id: Ifba04e0b70be29e5a82a67cf11837f740de57e32
Signed-off-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/26244
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Maintainer: Jason Lowe-Power <jason@lowepower.com>
Tested-by: kokoro <noreply+kokoro@google.com>

src/arch/arm/isa/insts/mem.isa

@@ -57,7 +57,7 @@ let {{
             # This shouldn't be part of the eaCode, but until the exec templates
             # are converted over it's the easiest place to put it.
             eaCode += '\n    unsigned memAccessFlags = '
-            eaCode += (string.join(memFlags, '|') + ';')
+            eaCode += ('|'.join(memFlags) + ';')
 
             codeBlobs["ea_code"] = eaCode