arch(7): small corrections for RISC-V

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.

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