Fix clock_t issue when compiling with newlib

[Chen-Yuanmeng]

Using newlib cross compilers (i.e. riscv64-unknown-elf-), the compiler gives the following error:

+ CC src/context.c
In file included from /home/kali/Desktop/ysyx-workbench/rt-thread-am/components/libc/compilers/common/include/sys/time.h:18,
                 from /home/kali/Desktop/ysyx-workbench/rt-thread-am/components/libc/compilers/common/include/sys/select.h:17,
                 from /opt/riscv/riscv64-unknown-elf/include/sys/types.h:50,
                 from /home/kali/Desktop/ysyx-workbench/rt-thread-am/bsp/abstract-machine/include/extra.h:3,
                 from ./rtconfig.h:3,
                 from /home/kali/Desktop/ysyx-workbench/rt-thread-am/include/rtthread.h:25,
                 from /home/kali/Desktop/ysyx-workbench/rt-thread-am/bsp/abstract-machine/src/context.c:3:
/opt/riscv/riscv64-unknown-elf/include/time.h:56:1: error: unknown type name 'clock_t'
   56 | clock_t    clock (void);
      | ^~~~~~~
/opt/riscv/riscv64-unknown-elf/include/time.h:33:1: note: 'clock_t' is defined in header '<time.h>'; this is probably fixable by adding '#include <time.h>'
   32 | #include <sys/_locale.h>
  +++ |+#include <time.h>
   33 | #endif
make: *** [/home/kali/Desktop/ysyx-workbench/abstract-machine/Makefile:101: /home/kali/Desktop/ysyx-workbench/rt-thread-am/bsp/abstract-machine/build/riscv64-nemu/src/context.o] Error 1

This is caused by Newlib's <sys/types.h> which includes <sys/select.h> before it defines clock_t.

[Copilot]

Pull request overview

This PR addresses a build failure with newlib-based cross compilers where clock_t is not yet defined when newlib’s <sys/types.h> pulls in <sys/select.h>, causing RT-Thread’s wrapper include chain to trigger <time.h> too early.

Changes:

• Adds a compiler/include-state conditional at the top of RT-Thread’s sys/select.h wrapper to fall back to the toolchain’s <sys/select.h> via #include_next during the problematic newlib include sequence.

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