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/riscv/isa/formats/mem.isa

@@ -73,7 +73,7 @@ def LoadStoreBase(name, Name, offset_code, ea_code, memacc_code, mem_flags,
 
     if mem_flags:
         mem_flags = [ 'Request::%s' % flag for flag in mem_flags ]
-        s = '\n\tmemAccessFlags = ' + string.join(mem_flags, '|') + ';'
+        s = '\n\tmemAccessFlags = ' + '|'.join(mem_flags) + ';'
         iop.constructor += s
 
     # select templates