Add SYS_riscv_flush_icache.

Fix the uapi import script to look at the riscv/asm/ directory too, and
re-run it to add the missing SYS_riscv_* entry (there's only this one)
to glibc-syscalls.h.

Test: treehugger
Change-Id: Ie52c6ca1943c05bb615932174e49e7fb79725a7b
diff --git a/libc/include/bits/glibc-syscalls.h b/libc/include/bits/glibc-syscalls.h
index c144919..79f7da0 100644
--- a/libc/include/bits/glibc-syscalls.h
+++ b/libc/include/bits/glibc-syscalls.h
@@ -906,6 +906,9 @@
 #if defined(__NR_restart_syscall)
   #define SYS_restart_syscall __NR_restart_syscall
 #endif
+#if defined(__NR_riscv_flush_icache)
+  #define SYS_riscv_flush_icache __NR_riscv_flush_icache
+#endif
 #if defined(__NR_rmdir)
   #define SYS_rmdir __NR_rmdir
 #endif