libroot/riscv64: Fix build. Add MIT optimized bswap from rv8

Change-Id: I4d0e1ec34d1568594cac0f7378f87852c997ab81
Reviewed-on: https://review.haiku-os.org/c/haiku/+/2166
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>

riscv64: Fill in more bulk architecture items around libroot/kernel

Change-Id: Ia2a86d8814d06950ea2d2d19d966c642d26f81d6
Reviewed-on: https://review.haiku-os.org/c/1302
Reviewed-by: Alex von Gluck IV <kallisti5@unixzen.com>
Reviewed-by: Adrien Destugues <pulkomandy@gmail.com>