Expose sbindir as a global Makefile variable

The sbindir is used in for some Python management utilities, but was
hardcoded rather than using the proper supplied variable. Exporting
from Makefile.global is a first step, next up is to fix all instances
where the directory name is hardcoded.

Discussion: https://github.com/greenplum-db/gpdb/pull/8179
Reviewed by Bradford Boyle, Kalen Krempely, Jamie McAtamney and
many more

src/Makefile.global.in

@@ -107,6 +107,7 @@ exec_prefix := @exec_prefix@
 datarootdir := @datarootdir@
 
 bindir := @bindir@
+sbindir := @sbindir@
 
 datadir := @datadir@
 ifeq "$(findstring pgsql, $(datadir))" ""