Clean up the riscv pmap_bootstrap() implementation.

• Build up phys_avail[] in a single loop, excluding memory used by the loaded kernel.
• Fix an array indexing bug in the aforementioned phys_avail[] initialization.[1]
• Remove some unneeded code copied from the arm64 implementation.

PR: 231515 [1]
Reviewed by: jhb
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D18464