Fix pg_bigm so that it can be registered properly in PostgreSQL 9.5 or before.

Fix pg_bigm so that it can be registered properly in PostgreSQL 9.5 or before.

Commit 0b09e86 introduced the bug which caused "CREATE EXTENSION pg_bigm" to
fail in PostgreSQL 9.5 or before. The cause of this failure is that
pg_bigm--1.2.sql included the DO command which tried to mark the functions
with PARALLEL SAFE label which those versions cannot handle. To prevent
those versions from emitting syntax failure when they look at PARALLEL SAFE
label, this commit changes so that "ALTER FUNCTION ... PARALLEL SAFE" command
is executed via EXECUTE command.