libbase, the OpenTitan Standard Library

This subtree provides headers and libraries known collectively as libbase, which serve as OpenTitan's ersatz libc.

Differences from a libc

libbase is not a libc, even though it implements a number of libc symbols, and should not be used as such. libbase is, rather, a place for base libraries which may need to make use of more compiler/platform intrinsics than average, in order to present a safe, stable, and uesful interface that other parts of OpenTitan can rely on.

In general, a library exposing general utilities for working close to the hardware should live in this subtree: for example, a library providing memcpy and related symbols. A library for talking to a particular peripheral, like a UART port, is a non-example.