include: sbi: Optimize reads of mhartid and mscratch

csr_read() is marked as volatile and clobbering memory, which is
generally the safe thing to do. However, these two CSRs do not have any
side effects, and the values returned do not change between calls. The
compiler can generate better code if we allow it to reorder calls to
these functions and cache the return value. Introduce csr_read_relaxed()
for this use case.

Signed-off-by: Samuel Holland <samuel.holland@sifive.com>
Reviewed-by: Anup Patel <anup@brainfault.org>

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