porting this RA6T2 micro-ros library to RX26T

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## Issue template

- Hardware description:
- RTOS:
- Installation type:
- Version or commit hash:

#### Steps to reproduce the issue

I did following changes in library_generation.sh
1、change arm-none-eabi-gcc to rx-elf-gcc
in libmicroros.mk
1、change
X_CC := arm-none-eabi-gcc
X_CXX := arm-none-eabi-g++
into
X_CC := rx-elf-gcc
X_CXX := rx-elf-g++
#### Expected behavior
generate a micro ros library

#### Actual behavior
build error
--- stderr: rcutils
/home/feng/RoboticHandProject/MicroROS_RX26T2/micro_ros_renesas2estudio_component/libmicroros/micro_ros_src/src/rcutils/src/time_unix.c: In function ‘rcutils_system_time_now’:
/home/feng/RoboticHandProject/MicroROS_RX26T2/micro_ros_renesas2estudio_component/libmicroros/micro_ros_src/src/rcutils/src/time_unix.c:65:7: error: implicit declaration of function ‘clock_gettime’ [-Wimplicit-function-declaration]
65 | if (clock_gettime(CLOCK_REALTIME, ×pec_now) < 0) {
| ^~~~~~~~~~~~~
gmake[5]: *** [CMakeFiles/rcutils.dir/build.make:446: CMakeFiles/rcutils.dir/src/time_unix.c.obj] Error 1
gmake[4]: *** [CMakeFiles/Makefile2:192: CMakeFiles/rcutils.dir/all] Error 2
gmake[3]: *** [Makefile:146: all] Error 2
---
Failed <<< rcutils [4.29s, exited with code 2]
make[2]: *** [libmicroros.mk:80: /home/feng/RoboticHandProject/MicroROS_RX26T2/micro_ros_renesas2estudio_component/library_generation/../libmicroros/micro_ros_src/install] Error 2
make[1]: [makefile:118: pre-build] Error 2 (ignored)
Finished <<< ament_cmake_export_definitions [0.52s]
#### Additional information

Start with library_generation.sh and libmicroros.mk to understand the RA6T2-to-RX26T toolchain changes, then inspect rcutils/src/time_unix.c where the build fails on clock_gettime. Reproduce the library generation with rx-elf-gcc and determine the project-supported resolution for the missing function. Done means the micro-ROS library generates successfully for RX26T without the rcutils build error.

Written by the indexing model from the issue text.