Make `MState::allocation_start` read more than one revocation bit at a time

[rmn30]

There is a loop in MState::allocation_start that reads a single revocation / shadow bit at a time to find the start of an allocation:

cheriot-rtos/sdk/core/allocator/alloc.h

Line 1478 in 0a02db8

while (!revoker.shadow_bit_get(address) && (address > base))

. This is inefficient and could be improved by reading a whole word of bits and using ctz and other tricks. Probably not a huge deal but worth investigating when we need to improve performance of heap_free.

[rmn30]

As @davidchisnall points out in many cases this won't be necessary as the address should already point at the start of the allocation. For claims it may be a sub allocation in which case we'll have to iterate backwards to find the start. Not sure if that's worth optimising.