pw_rpc: Work around bug in GCC 8
pw_rpc: Work around bug in GCC 8

This works around a GCC bug that was fixed in GCC 9
(https://gcc.gnu.org/bugzilla/show_bug.cgi?id=86678). GCC 8 incorrectly
reports a "call to non-'constexpr' function" error in some valid
constexpr functions.

Change-Id: Iab1fda0c6fcf5ca7e428899523fc51f8acb42876
Reviewed-on: https://pigweed-review.googlesource.com/c/pigweed/pigweed/+/85900
Pigweed-Auto-Submit: Wyatt Hepler <cca3bedb37979eacd157a9bceb473582df8aea1e@google.com>
Reviewed-by: Alexei Frolov <a1b001416a8cf9aaea32a3411f8f3c0793ebc0b5@google.com>
Commit-Queue: Auto-Submit <14637eda5603879df170705285bf30f5996692f0@pigweed.google.com.iam.gserviceaccount.com>

google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed,google/pigweed