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/kernel/tools/update_all.py b/libc/kernel/tools/update_all.py
index abc72a4..ae89a80 100755
--- a/libc/kernel/tools/update_all.py
+++ b/libc/kernel/tools/update_all.py
@@ -92,6 +92,7 @@
                      'kernel/uapi/asm-arm/asm/unistd.h',
                      'kernel/uapi/asm-arm/asm/unistd-eabi.h',
                      'kernel/uapi/asm-arm/asm/unistd-oabi.h',
+                     'kernel/uapi/asm-riscv/asm/unistd.h',
                      'kernel/uapi/asm-x86/asm/unistd_32.h',
                      'kernel/uapi/asm-x86/asm/unistd_64.h',
                      'kernel/uapi/asm-x86/asm/unistd_x32.h']: