[NFC][RISCV][builtins] Remove some hard-coded values from i-cache clear routine

Remove some hard-coded values from i-cache clear routine

Differential Revision: https://reviews.llvm.org/D87578

compiler-rt/lib/builtins/clear_cache.c

@@ -46,6 +46,11 @@ uintptr_t GetCurrentProcess(void);
 #include <unistd.h>
 #endif
 
+#if defined(__linux__) && defined(__riscv)
+// to get platform-specific syscall definitions
+#include <linux/unistd.h>
+#endif
+
 // The compiler generates calls to __clear_cache() when creating
 // trampoline functions on the stack for use with nested functions.
 // It is expected to invalidate the instruction cache for the
@@ -148,9 +153,10 @@ void __clear_cache(void *start, void *end) {
   for (uintptr_t dword = start_dword; dword < end_dword; dword += dword_size)
     __asm__ volatile("flush %0" : : "r"(dword));
 #elif defined(__riscv) && defined(__linux__)
-#define __NR_riscv_flush_icache (244 + 15)
+  // See: arch/riscv/include/asm/cacheflush.h, arch/riscv/kernel/sys_riscv.c
   register void *start_reg __asm("a0") = start;
   const register void *end_reg __asm("a1") = end;
+  // "0" means that we clear cache for all threads (SYS_RISCV_FLUSH_ICACHE_ALL)
   const register long flags __asm("a2") = 0;
   const register long syscall_nr __asm("a7") = __NR_riscv_flush_icache;
   __asm __volatile("ecall"