In commit d42b9ff, an optimization was introduced that can speed up
Regexp#match by 15% when it matches with strings of different encodings.
This optimization, however, does not work across ractors. To fix this,
we only use the optimization if no ractors have been started. In the
future, we could use atomics for the reference counting if we find it's
needed and if it's more performant.

The backtrace of the misbehaving native thread:

```
  * frame #0: 0x0000000189c94388 libsystem_kernel.dylib`__pthread_kill + 8
    frame ruby#1: 0x0000000189ccd88c libsystem_pthread.dylib`pthread_kill + 296
    frame ruby#2: 0x0000000189bd6c60 libsystem_c.dylib`abort + 124
    frame ruby#3: 0x0000000189adb174 libsystem_malloc.dylib`malloc_vreport + 892
    frame ruby#4: 0x0000000189adec90 libsystem_malloc.dylib`malloc_report + 64
    frame ruby#5: 0x0000000189ae321c libsystem_malloc.dylib`___BUG_IN_CLIENT_OF_LIBMALLOC_POINTER_BEING_FREED_WAS_NOT_ALLOCATED + 32
    frame ruby#6: 0x00000001001c3be4 ruby`onig_free_body(reg=0x000000012d84b660) at regcomp.c:5663:5
    frame ruby#7: 0x00000001001ba828 ruby`rb_reg_prepare_re(re=4748462304, str=4748451168) at re.c:1680:13
    frame ruby#8: 0x00000001001bac58 ruby`rb_reg_onig_match(re=4748462304, str=4748451168, match=(ruby`reg_onig_search [inlined] rbimpl_RB_TYPE_P_fastpath at value_type.h:349:14
ruby`reg_onig_search [inlined] rbimpl_rstring_getmem at rstring.h:391:5
ruby`reg_onig_search at re.c:1781:5), args=0x000000013824b168, regs=0x000000013824b150) at re.c:1708:20
    frame ruby#9: 0x00000001001baefc ruby`rb_reg_search_set_match(re=4748462304, str=4748451168, pos=<unavailable>, reverse=0, set_backref_str=1, set_match=0x0000000000000000) at re.c:1809:27
    frame ruby#10: 0x00000001001bae80 ruby`rb_reg_search0(re=<unavailable>, str=<unavailable>, pos=<unavailable>, reverse=<unavailable>, set_backref_str=<unavailable>, match=<unavailable>) at re.c:1861:12 [artificial]
    frame ruby#11: 0x0000000100230b90 ruby`rb_pat_search0(pat=<unavailable>, str=<unavailable>, pos=<unavailable>, set_backref_str=<unavailable>, match=<unavailable>) at string.c:6619:16 [artificial]
    frame ruby#12: 0x00000001002287f4 ruby`rb_str_sub_bang [inlined] rb_pat_search(pat=4748462304, str=4748451168, pos=0, set_backref_str=1) at string.c:6626:12
    frame ruby#13: 0x00000001002287dc ruby`rb_str_sub_bang(argc=1, argv=0x00000001381280d0, str=4748451168) at string.c:6668:11
    frame ruby#14: 0x000000010022826c ruby`rb_str_sub
```

You can reproduce this by running:
```
RUBY_TESTOPTS="--name=/test_str_capitalize/" make test-all TESTS=test/ruby/test_m17n.comb
```

However, you need to run it with multiple ractors at once.

Co-authored-by: jhawthorn <john@hawthorn.email>