porting this RA6T2 micro-ros library to RX26T

先从 library_generation.sh 和 libmicroros.mk 入手，了解 RA6T2-to-RX26T toolchain 的变更，然后检查 rcutils/src/time_unix.c，其中 build 会在 clock_gettime 处失败。使用 rx-elf-gcc 重现库生成过程，并确定项目支持的缺失函数解决方案。当 micro-ROS 库能够在 RX26T 上成功生成且没有 rcutils build 错误时，即视为完成。

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, &timespec_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