Make embedded string length a long for VWA

A short (2 bytes) will cause unaligned struct accesses when strings are
used as a buffer to directly store binary data.

include/ruby/internal/core/rstring.h

@@ -280,7 +280,7 @@ struct RString {
         /** Embedded contents. */
         struct {
 #if USE_RVARGC
-            unsigned short len;
+            long len;
             /* This is a length 1 array because:
              *   1. GCC has a bug that does not optimize C flexible array members
              *      (https://gcc.gnu.org/bugzilla/show_bug.cgi?id=102452)

test/-ext-/string/test_capacity.rb

@@ -66,7 +66,7 @@ class Test_StringCapacity < Test::Unit::TestCase
 
   def embed_header_size
     if GC.using_rvargc?
-      2 * RbConfig::SIZEOF['void*'] + RbConfig::SIZEOF['short']
+      2 * RbConfig::SIZEOF['void*'] + RbConfig::SIZEOF['long']
     else
       2 * RbConfig::SIZEOF['void*']
     end