Add __riscv_flush_icache() to <sys/cachectl.h>.

The obsolete mips header rides again!

The most interesting part of this change is that I've removed the hack
that meant that all system call wrappers starting with `__` defaulted to
being hidden symbols. That's no longer useful given our linker scripts,
and it actively got in the way here because the public libc symbol
actually starts with `__` in glibc, and it would be weird and annoying
for developers if we chose a different name.

Test: strace
Change-Id: I230479787895e8e34f566ade36346a8241eea998
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index c69e609..41d18a6 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1589,6 +1589,7 @@
     localtime_rz;
     mbsrtowcs_l;
     mktime_z;
+    __riscv_flush_icache; # riscv64
     timespec_getres;
     tzalloc;
     tzfree;