transform_mjit_header.rb: accept having attribute

between a type of return value and function identifier. Like:

```
rb_control_frame_t *
__attribute__ ((__fastcall__)) rb_vm_opt_struct_aref(rb_execution_context_t *ec, rb_control_frame_t *reg_cfp)
{
    ...
}
```

[Bug #14949]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@64138 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

tool/transform_mjit_header.rb

@@ -10,7 +10,11 @@ PROGRAM = File.basename($0, ".*")
 module MJITHeader
   ATTR_VALUE_REGEXP  = /[^()]|\([^()]*\)/
   ATTR_REGEXP        = /__attribute__\s*\(\(#{ATTR_VALUE_REGEXP}*\)\)/
-  FUNC_HEADER_REGEXP = /\A(\s*#{ATTR_REGEXP})*[^\[{(]*\((#{ATTR_REGEXP}|[^()])*\)(\s*#{ATTR_REGEXP})*\s*/
+  # Example:
+  #   VALUE foo(int bar)
+  #   VALUE __attribute__ ((foo)) bar(int baz)
+  #   __attribute__ ((foo)) VALUE bar(int baz)
+  FUNC_HEADER_REGEXP = /\A((\s*#{ATTR_REGEXP})|[^\[{(])*\((#{ATTR_REGEXP}|[^()])*\)(\s*#{ATTR_REGEXP})*\s*/
   TARGET_NAME_REGEXP = /\A(rb|ruby|vm|insn|attr)_/
 
   # Predefined macros for compilers which are already supported by MJIT.