[Bug #21793]

To fix a naming conflict on solaris.

Allows to remove some duplicated code like szqueue_length, etc.

If the queue was allocated without calling initialize,
`ary` will be `0`.

We receive the ec as argument, it's much cheaper to pass it
around that to look it up again.

We never need the actual thread object and this avoid any issue
if the thread object is ever moved.

Since it now live in the EC.

Mutexes spend a significant amount of time in `rb_fiber_serial`
because it can't be inlined (except with LTO).
The fiber struct is opaque the so function can't be defined as inlineable.

Ideally the while fiber struct would not be opaque to the rest of
Ruby core, but it's tricky to do.

Instead we can store the fiber serial in the execution context
itself, and make its access cheaper:

```
$ hyperfine './miniruby-baseline --yjit /tmp/mut.rb' './miniruby-inline-serial --yjit /tmp/mut.rb'
Benchmark 1: ./miniruby-baseline --yjit /tmp/mut.rb
  Time (mean ± σ):      4.011 s ±  0.084 s    [User: 3.977 s, System: 0.011 s]
  Range (min … max):    3.950 s …  4.245 s    10 runs

Benchmark 2: ./miniruby-inline-serial --yjit /tmp/mut.rb
  Time (mean ± σ):      3.495 s ±  0.150 s    [User: 3.448 s, System: 0.009 s]
  Range (min … max):    3.340 s …  3.869 s    10 runs

Summary
  ./miniruby-inline-serial --yjit /tmp/mut.rb ran
    1.15 ± 0.05 times faster than ./miniruby-baseline --yjit /tmp/mut.rb
```

```ruby
i = 10_000_000
mut = Mutex.new
while i > 0
  i -= 1
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
  mut.synchronize { }
end
```

Saves one more call to GET_EC()

It's more consistent with Mutex, but also the `#wait` method
benefits from receiving the execution context instead of having
to call `GET_EC`.

That call is surprisingly expensive, so trying doing it once
in `#synchronize` and then passing the EC to lock and unlock
saves quite a few cycles.

Before:

```
ruby 4.0.0dev (2025-12-10T09:30:18Z master c5608ab4d7) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               Mutex     1.888M i/100ms
             Monitor     1.633M i/100ms
Calculating -------------------------------------
               Mutex     22.610M (± 0.2%) i/s   (44.23 ns/i) -    113.258M in   5.009097s
             Monitor     19.148M (± 0.3%) i/s   (52.22 ns/i) -     96.366M in   5.032755s
```

After:
```
ruby 4.0.0dev (2025-12-10T10:40:07Z speedup-mutex 1c901cd4f8) +YJIT +PRISM [arm64-darwin25]
Warming up --------------------------------------
               Mutex     2.095M i/100ms
             Monitor     1.578M i/100ms
Calculating -------------------------------------
               Mutex     24.456M (± 0.4%) i/s   (40.89 ns/i) -    123.584M in   5.053418s
             Monitor     19.176M (± 0.1%) i/s   (52.15 ns/i) -     96.243M in   5.018977s
```

Bench:

```
require 'bundler/inline'

gemfile do
  gem "benchmark-ips"
end

mutex = Mutex.new
require "monitor"
monitor = Monitor.new

Benchmark.ips do |x|
  x.report("Mutex") { mutex.synchronize { } }
  x.report("Monitor") { monitor.synchronize { } }
end
```