riscv64: add <sys/hwprobe.h>.

This is source compatible with the current proposal for glibc.

Bug: https://github.com/google/android-riscv64/issues/27
Test: treehugger
Change-Id: I428777e4eac1fe643d442a93a4b3ad1fdf0ffd97
diff --git a/libc/libc.map.txt b/libc/libc.map.txt
index 86c8288..e90618d 100644
--- a/libc/libc.map.txt
+++ b/libc/libc.map.txt
@@ -1590,6 +1590,7 @@
     mbsrtowcs_l;
     mktime_z;
     __riscv_flush_icache; # riscv64
+    __riscv_hwprobe; # riscv64
     timespec_getres;
     tzalloc;
     tzfree;