README: Update toolchain information

Recent FW_PIC=y changes actually require toolchains with PIE support
and it is on by default. Existing doc uses a GNU 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.

Also indicate that "riscv64-unknown-freebsd-" as an alternative to
"riscv64-linux-" GNU toolchain.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>