Upgrade to gRPC 1.27.0 breaks non-glibc Linux

With the upgrade to gRPC 1.27.0 in !1960 (merged), a new version of google-protobuf (v3.11.4) is pulled in. Turns out that starting with 63f324a99 (Roll forward Ruby upb changes now that protobuf Ruby build is fixed (#5866), 2019-08-14), included since v3.10.0, protobuf's Ruby extension has introduced use of the __va_copy symbol. As that's a non-standard symbol exposed by glibc, only, the update to google-protobuf 3.11.4 breaks non-glibc Linux like Alpine Linux.