Mutex: avoid repeated calls to `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
```

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