[build] fix the -Wunused-command-line-argument warning on macOS (#8839)

Use the command `make -f src/posix/Makefile-posix` to build the posix
platform on macOS, it generates the `-Wunused-command-line-argument`
warning (refer to #8828).

This commit moves the linker option `-rdynamic` from `COMMONCFLAGS` to
the `LDFLAGS` to resolve the warning.

src/posix/Makefile-posix

@@ -78,7 +78,6 @@ UPTIME                               ?= 1
 
 COMMONCFLAGS                         := \
     -g                                  \
-    -rdynamic                           \
     $(NULL)
 
 # If the user has asserted COVERAGE, alter the configuration options
@@ -144,6 +143,7 @@ CXXFLAGS                       += \
 
 LDFLAGS                        += \
     $(COMMONCFLAGS)               \
+    -rdynamic                     \
     $(NULL)
 
 TopSourceDir                   := $(dir $(shell readlink $(firstword $(MAKEFILE_LIST))))../..