tool headers nolibc: add RISCV support

This adds support for the RISCV architecture (32 and 64 bit) to the
nolibc header file.

Signed-off-by: Pranith Kumar <bobby.prani@gmail.com>
[willy: minimal rewording of the commit message]
Signed-off-by: Willy Tarreau <w@1wt.eu>