[PATCH] README: Update toolchain information

[Jessica Clarke]

On 11 Jul 2021, at 17:32, Bin Meng <bmeng.cn at gmail.com> wrote:
> 
> Recent FW_PIC=y changes actually require toolchains with PIE support
> and it is on by default. Existing doc uses a bare-metal toolchain as
> examples but it does not support PIE.
> 
> Replace references of bare-metal toolchain prefix with linux toolchain
> prefix everywhere in documentation.

LLVM toolchains should continue to prefer bare-metal triples. Please
also include riscv64-unknown-freebsd- as an alternative to
riscv64-linux-gnu- so there is no bias towards Linux, as is often
unfortunately seen in the RISC-V world.

Jess