misc: Rename the (De|En)codeVarint function _(De|En)codeVarint32.

misc: Rename the (De|En)codeVarint function _(De|En)codeVarint32.

DecodeVarint implicitly truncates its inputs to 32 bits, and that should be
obvious from its name, and so not a surprise to the caller. EncodeVarint also
has the suffix added for consistency. Both functions are only used in
protolib.py, so add a _ to make it private (in the python sense) to the
module.

Change-Id: I835295717bb51672351b963fe1227ed619836f22
Reviewed-on: https://gem5-review.googlesource.com/5001
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>