riscv: Mark __se_sys_* functions __used

Clang doesn't think ___se_sys_* functions used even though they are
aliased to __se_sys_*, resulting in -Wunused-function warnings when
building rv32. For example:

   mm/oom_kill.c:1195:1: warning: unused function '___se_sys_process_mrelease' [-Wunused-function]
    1195 | SYSCALL_DEFINE2(process_mrelease, int, pidfd, unsigned int, flags)
         | ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/syscalls.h:221:36: note: expanded from macro 'SYSCALL_DEFINE2'
     221 | #define SYSCALL_DEFINE2(name, ...) SYSCALL_DEFINEx(2, _##name, __VA_ARGS__)
         |                                    ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   include/linux/syscalls.h:231:2: note: expanded from macro 'SYSCALL_DEFINEx'
     231 |         __SYSCALL_DEFINEx(x, sname, __VA_ARGS__)
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   arch/riscv/include/asm/syscall_wrapper.h:81:2: note: expanded from macro '__SYSCALL_DEFINEx'
      81 |         __SYSCALL_SE_DEFINEx(x, sys, name, __VA_ARGS__)                         \
         |         ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
   arch/riscv/include/asm/syscall_wrapper.h:40:14: note: expanded from macro '__SYSCALL_SE_DEFINEx'
      40 |         static long ___se_##prefix##name(__MAP(x,__SC_LONG,__VA_ARGS__))
         |                     ^~~~~~~~~~~~~~~~~~~~
   <scratch space>:30:1: note: expanded from here
      30 | ___se_sys_process_mrelease
         | ^~~~~~~~~~~~~~~~~~~~~~~~~~
   1 warning generated.

Mark the functions __used explicitly to fix the Clang warnings.

Fixes: a9ad7329

 ("riscv: Fix syscall wrapper for >word-size arguments")
Reported-by: Linux Kernel Functional Testing <lkft@linaro.org>
Tested-by: Linux Kernel Functional Testing <lkft@linaro.org>
Signed-off-by: Sami Tolvanen <samitolvanen@google.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Conor Dooley <conor.dooley@microchip.com>
Link: https://lore.kernel.org/r/20240326153712.1839482-2-samitolvanen@google.com


Signed-off-by: Palmer Dabbelt <palmer@rivosinc.com>