fix SEGV on clang-16/18

Maybe because of TLS/coroutine problem, CI fails on clang-16/18

```
  1) Failure:
TestTimeout#test_ractor [/tmp/ruby/src/trunk_clang_18/test/test_timeout.rb:288]:
pid 307341 killed by SIGSEGV (signal 11) (core dumped)
| /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:98: [BUG] Segmentation fault at 0x0000000000000030
| ruby 4.0.0dev (2025-12-07T16:51:02Z master 4f900c3) +PRISM [x86_64-linux]
|
| -- Control frame information -----------------------------------------------
| c:0006 p:---- s:0026 e:000025 l:y b:---- CFUNC  :sleep
| c:0005 p:---- s:0023 e:000022 l:y b:---- CFUNC  :wait
| c:0004 p:0020 s:0017 e:000016 l:n b:---- BLOCK  /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:98 [FINISH]
| c:0003 p:---- s:0014 e:000013 l:y b:---- CFUNC  :synchronize
| c:0002 p:0072 s:0010 e:000009 l:n b:---- BLOCK  /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:96 [FINISH]
| c:0001 p:---- s:0003 e:000002 l:y b:---- DUMMY  [FINISH]
|
| -- Ruby level backtrace information ----------------------------------------
| /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:96:in 'block in create_timeout_thread'
| /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:96:in 'synchronize'
| /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:98:in 'block (2 levels) in create_timeout_thread'
| /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:98:in 'wait'
| /tmp/ruby/src/trunk_clang_18/lib/timeout.rb:98:in 'sleep'
|
| -- Threading information ---------------------------------------------------
| Total ractor count: 3
| Ruby thread count for this ractor: 2
|
| -- Machine register context ------------------------------------------------
|  RIP: 0x0000602b1e08a5b5 RBP: 0x000071c65facd130 RSP: 0x000071c6258842e0
|  RAX: 0x0000000000000000 RBX: 0x000000006935f7c4 RCX: 0x0000000000000000
|  RDX: 0x0000602b1e520c20 RDI: 0x000071c620012480 RSI: 0x000071c620012480
|   R8: 0x0000000000000000  R9: 0x0000000000000000 R10: 0x0000000000000000
|  R11: 0x0000000000000000 R12: 0x000071c65fa2f640 R13: 0x000071c65fb66e48
|  R14: 0x0000000000000000 R15: 0xfdccaa3270000002 EFL: 0x0000000000010202
|
| -- C level backtrace information -------------------------------------------
| /tmp/ruby/build/trunk_clang_18/ruby(rb_print_backtrace+0x14) [0x602b1e31a6ea] /tmp/ruby/src/trunk_clang_18/vm_dump.c:1105
| /tmp/ruby/build/trunk_clang_18/ruby(rb_vm_bugreport) /tmp/ruby/src/trunk_clang_18/vm_dump.c:1450
| /tmp/ruby/build/trunk_clang_18/ruby(rb_bug_for_fatal_signal+0x15c) [0x602b1e2d960c] /tmp/ruby/src/trunk_clang_18/error.c:1131
| /tmp/ruby/build/trunk_clang_18/ruby(sigsegv+0x5a) [0x602b1e05528a] /tmp/ruby/src/trunk_clang_18/signal.c:948
| /lib/x86_64-linux-gnu/libc.so.6(0x71c65fd46320) [0x71c65fd46320]
| /tmp/ruby/build/trunk_clang_18/ruby(vm_check_ints_blocking+0x0) [0x602b1e08a5b5] /tmp/ruby/src/trunk_clang_18/vm_core.h:2097
| /tmp/ruby/build/trunk_clang_18/ruby(rb_current_execution_context) /tmp/ruby/src/trunk_clang_18/thread_sync.c:617
| /tmp/ruby/build/trunk_clang_18/ruby(rb_mutex_sleep) /tmp/ruby/src/trunk_clang_18/thread_sync.c:617
```

This patch introduces workaround by acquiring EC before swithcing coroutine.

Author: ko1

thread_sync.c

@@ -599,6 +599,8 @@ mutex_sleep_begin(VALUE _arguments)
 VALUE
 rb_mutex_sleep(VALUE self, VALUE timeout)
 {
+    rb_execution_context_t *ec = GET_EC();
+
     if (!NIL_P(timeout)) {
         // Validate the argument:
         rb_time_interval(timeout);
@@ -614,7 +616,7 @@ rb_mutex_sleep(VALUE self, VALUE timeout)
 
     VALUE woken = rb_ensure(mutex_sleep_begin, (VALUE)&arguments, mutex_lock_uninterruptible, self);
 
-    RUBY_VM_CHECK_INTS_BLOCKING(GET_EC());
+    RUBY_VM_CHECK_INTS_BLOCKING(ec);
     if (!woken) return Qnil;
     time_t end = time(0) - beg;
     return TIMET2NUM(end);