lang/ruby{26,27}: work around clang 12 -Wcompound-token-split-by-macro warning

During an exp-run for llvm 12 (see bug 255570), it turned out that
several ruby gem extensions do not build with clang 12.0.0, for example
devel/rubygem-thrift:

compiling binary_protocol_accelerated.c
binary_protocol_accelerated.c:404:68: error: '(' and '{' tokens introducing statement expression appear in different macro expansion contexts [-Werror,-Wcompound-token-split-by-macro]

VALUE thrift_binary_protocol_class = rb_const_get(thrift_module, rb_intern("BinaryProtocol"));
                                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/ruby-2.7/ruby/ruby.h:1847:23: note: expanded from macro 'rb_intern'

__extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
              ^

binary_protocol_accelerated.c:404:68: note: '{' token is here

VALUE thrift_binary_protocol_class = rb_const_get(thrift_module, rb_intern("BinaryProtocol"));
                                                                 ^~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/ruby-2.7/ruby/ruby.h:1847:24: note: expanded from macro 'rb_intern'

__extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/ruby-2.7/ruby/ruby.h:1832:5: note: expanded from macro 'RUBY_CONST_ID_CACHE'

{                                                   \
^

and similarly www/unit-ruby:

src/ruby/nxt_ruby.c:242:21: error: '(' and '{' tokens introducing statement expression appear in different macro expansion contexts [-Werror,-Wcompound-token-split-by-macro]

nxt_ruby_call = rb_intern("call");
                ^~~~~~~~~~~~~~~~~

/usr/local/include/ruby-2.7/ruby/ruby.h:1847:23: note: expanded from macro 'rb_intern'

__extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
              ^

src/ruby/nxt_ruby.c:242:21: note: '{' token is here

nxt_ruby_call = rb_intern("call");
                ^~~~~~~~~~~~~~~~~

/usr/local/include/ruby-2.7/ruby/ruby.h:1847:24: note: expanded from macro 'rb_intern'

__extension__ (RUBY_CONST_ID_CACHE((ID), (str))) : \
               ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

/usr/local/include/ruby-2.7/ruby/ruby.h:1832:5: note: expanded from macro 'RUBY_CONST_ID_CACHE'

{                                                   \
^

The gem extensions appear to purposefully compile using -Werror, and
this new -Wcompound-token-split-by-macro is enabled by default in clang
12 and later. Of course we could go over all these extensions, and
either remove -Werror or add -Wno-compound-token-split-by-macro, but
that seems quite a lot of effort.

Instead, I have submitted a pull request to ruby's GitHub, which fixes
this by modifying the main ruby.h header:

• Add RUBY_CONST_ID_CACHE_NB() (i.e. no-brace) which contains the code itself, without any braces
• RUBY_CONST_ID_CACHE() which uses RUBY_CONST_ID_CACHE_NB(), but puts braces around it (so no existing code using this macro breaks)
• Finally, change rb_intern() so the extension directly creates a gcc statement expression, using the RUBY_CONST_ID_CACHE_NB() macro

Patch this locally in our lang/ruby26 and lang/ruby27 ports for now,
until upstream manages to get this in.

Approved by: maintainer timeout (2 weeks)
PR: 255910
MFH: 2021Q2