Starting with GCC 7 and clang 15, single-bit operations such as
fetch_or(1) & 1 are translated into 80386 instructions such as
LOCK BTS, instead of using the generic translation pattern
of emitting a loop around LOCK CMPXCHG.

Given that the oldest currently supported GNU/Linux distributions
ship GCC 7, and that older versions of GCC are out of support,
let us remove some work-arounds that are not strictly necessary.
If someone compiles the code using an older compiler, it will work
but possibly less efficiently.

srw_mutex_impl::HOLDER: Changed from 1U<<31 to 1 in order to
work around https://github.com/llvm/llvm-project/issues/37322
which is specific to setting the most significant bit.

srw_mutex_impl::WAITER: A multiplier of waiting requests.
This used to be 1, which would now collide with HOLDER.

fil_space_t::set_stopping(): Remove this unused function.

In MSVC we need _interlockedbittestandset() for LOCK BTS.