cpu: Use std::abs() in traffic_gen.cc.
cpu: Use std::abs() in traffic_gen.cc.

When building with clang with the --without-tcmalloc flag set, the
-fno-builtin flag is not used, and clang can then detect that the
integer version of abs(), apparently the C version, is being used on a
floating point value in traffic_gen.cc.

This change takes clang's suggestion to use std::abs instead, and also
includes a header file which will provide it.

Change-Id: Ic28ed7454b2ac00c89328d9d0314aed74e946643
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/41597
Reviewed-by: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Maintainer: Andreas Sandberg <c9f5686ee5bd561c93f9de79681eee75bfa7f36e@arm.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>

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