Looks like we were missing an "extern" in two places. While I was at it,
also tidy up guc_gp.c by moving the definition of Debug_resource_group
into cdbvars.c, and add declaration of
gp_encoding_check_locale_compatibility to cdbvars.h.

This is uncovered by building with GCC 10 and Clang 11, where
-fno-common is the new default [1][2] (vis a vis -fcommon). I could also
reproduce this by turning on "-fno-common" in older releases of GCC and
Clang.

We were relying on a myth (or legacy compiler behavior, rather) that C
tentative definitions act _just like_ declarations -- in plain English:
missing an "extern" in a global variable declaration-wannabe wouldn't
harm you, as long as you don't put an initial value after it.

This resolves #10072.

[1] "3.17 Options for Code Generation Conventions: -fcommon"
https://gcc.gnu.org/onlinedocs/gcc-10.1.0/gcc/Code-Gen-Options.html#index-tentative-definitions
[2] "Porting to GCC 10" https://gcc.gnu.org/gcc-10/porting_to.html
[3] "[Driver] Default to -fno-common for all targets" https://reviews.llvm.org/D75056

(cherry picked from commit ee7eb0e8)