fix: fix build errors with newer GCC and resolve const qualifier mism…

[Dirinkbottle]

…atch with newer GCC #12
修复在 x86_64 GCC 16.2.1 20260810 下编译 ARCH=native 时出现的兼容性问题。

1. 修复 cstring.c 中字符串函数返回值的 const qualifier mismatch，避免新版 GCC 在严格类型检查下触发 -Werror=discarded-qualifiers。

2. 修复项目兼容层与宿主 libc 头文件之间的类型/声明不一致，以及新版 GCC 更严格检查所暴露出的相关编译错误，使 bsp/abstract-machine 下的 make ARCH=native 能正常通过编译。

[KEDIT2007]

我遇到了同样的问题，拉取这个 PR 后 ARCH=native 可以正常编译，但 ARCH=$ISA-nemu 不行。

我的交叉编译依赖 newlib。newlib 的 sys/types.h 在定义 clock_t / time_t 之前就会 #include <sys/select.h>。这个包含会被解析到 RT-Thread 的 sys/select.h，再间接包含 newlib 的 time.h，而此时 clock_t 尚未定义。glibc 没有这个问题。

部分报错：

In file included from .../sys/time.h:18,
                 from .../sys/select.h:17,
                 from .../newlib/include/sys/types.h:50,
                 from .../bsp/abstract-machine/include/extra.h:2,
                 from ./rtconfig.h:3,
                 from .../include/rtthread.h:25,
                 from .../bsp/abstract-machine/src/context.c:3:
.../newlib/include/time.h:56:1: error: unknown type name ‘clock_t’
   56 | clock_t    clock (void);
      | ^~~~~~~

把对 sys/types.h 的包含限制在 native 之后，我这边 nemu 和 native 都能编译：

#ifdef __ISA_NATIVE__
#include <sys/types.h>
/* posix remaps ... */
#else
typedef long suseconds_t;
typedef unsigned long useconds_t;
#endif

[Dirinkbottle]

ISA: riscv32 PLATFORM: nemu
- CC /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/filesystems/devfs/devfs.c
- CC /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/filesystems/elmfat/dfs_elm.c
- CC /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/filesystems/elmfat/ff.c
- CC /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/filesystems/elmfat/ffunicode.c
- CC /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/filesystems/romfs/dfs_romfs.c
In file included from /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/libc/compilers/common/include/sys/time.h:18,
                 from /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/libc/compilers/common/include/sys/select.h:17,
                 from /opt/riscv-multilib-all/riscv64-unknown-elf/include/sys/types.h:50,
                 from /opt/riscv-multilib-all/riscv64-unknown-elf/include/sys/_default_fcntl.h:217,
                 from /opt/riscv-multilib-all/riscv64-unknown-elf/include/sys/fcntl.h:3,
                 from /opt/riscv-multilib-all/riscv64-unknown-elf/include/fcntl.h:1,
                 from /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/include/dfs.h:18,
                 from /home/inkbottle/othersrc/ysyx/ysyx-workbench/ics-pa/rt-thread-am/components/dfs/dfs_v1/filesystems/romfs/dfs_romfs.c:11:
/opt/riscv-multilib-all/riscv64-unknown-elf/include/time.h:56:1: error: unknown type name 'clock_t'
   56 | clock_t    clock (void);
      | ^~~~~~~
/opt/riscv-multilib-all/riscv64-unknown-elf/include/time.h:30:1: note: 'clock_t' is defined in header '<time.h>'; this is probably fixable by adding '#include <time.h>'
   29 | #include <sys/timespec.h>
  +++ |+#include <time.h>
   30 | 

尝试了一下,我这里最后又会回到sys/time.h去找struct timeval. 所以这次我手动补了struct timeval; 不包括完整定义. 并且修改了cstdlib在前面包含完整的stddef.h即可.后面需要完整的timeval再包含sys/time.h 此时time.h需要的类型已经被stddef.h补齐