Merged PR 1768243: Fix rhel8 packaging build

### Does this PR have any customer impact?
No

### Type (Feature, Refactoring, Bugfix, DevOps, Testing, Perf, etc)
Bugfix

### Does it involve schema level changes? (Table, Column, Index, UDF, etc level changes)
No
### Are you introducing any new config? If yes, do you have tests with and without them being set?
No
### ChangeLog (Refer [Template](../oss/CHANGELOG.md))

### Description

Rhel8 package build uses PG_CPPFLAGS not PG_CFLAGS in makefile

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#### AI description  (iteration 1)
#### PR Classification
Bug fix addressing RHEL8 packaging build issues.

#### PR Summary
This pull request resolves the RHEL8 packaging build problem by ensuring the atomic implementation flag is correctly applied for C++ compilation.
- In `oss/pg_documentdb/Makefile`, added `-DCROARING_ATOMIC_IMPL=3` to `PG_CPPFLAGS`.
<!-- GitOpsUserAgent=GitOps.Apps.Server.pullrequestcopilot -->

Author: visridha

pg_documentdb/Makefile

@@ -41,6 +41,7 @@ OBJS = $(patsubst %.c,%.o,$(SOURCES))
 
 # Update to notify roaring to use the C atomics implementation (as opposed to c++): See roaring.h CROARING_ATOMIC_IMPL_C
 PG_CFLAGS += -DCROARING_ATOMIC_IMPL=3
+PG_CPPFLAGS += -DCROARING_ATOMIC_IMPL=3
 
 DEBUG ?= no
 ifeq ($(DEBUG),yes)