base: Make the functions in intmath.hh constexpr.
base: Make the functions in intmath.hh constexpr.

These simple functions can potentially be evaluated at compile time, and
marking them constexpr makes them available in more contexts.

Change-Id: I9cf39c517e7c53c276883f311739c1b153ccfd44
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/42357
Maintainer: Gabe Black <d3e8afea784ab4a50a2dbf9149d2eaa8478e1cbc@gmail.com>
Tested-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: Daniel Carvalho <91779fc734d358d9575ec4910de29bfcbcaf1b22@yahoo.com.br>

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