arch(7): small corrections for RISC-V

Document that RISC-V supports multiple page sizes: 4K, 2M, and 1G.

RISC-V's long double is always 128-bits wide, therefore quad precision.

Mention __riscv_float_abi_soft, which can be used to differentiate between
riscv64 and riscv64sf in userland code.

MFC after: 3 days