Enable GC loading from a shared library

[eightbitraptor]

This PR replaces #10302 as a method for dynamically loading a shared GC library. This new approach has been guided by feedback from @kateinoigakukun and @nobu

The approach taken here is as follows:

• Enable this feature by configuring with --with-shared-gc
• When enabled, Ruby will check for the presence of the environment variable RUBY_GC_LIBRARY_PATH
• If that path exists and points to a valid shared object
  • Open the shared object (using dln_open), and map the Init_GC function to a function map
  • call Init_GC as part of rb_objspace_alloc when initialising the GC
• If that path does not point to a valid shared object
  • Degrade gracefully to initialising the existing Ruby objspace and GC

When Ruby is configured without the --with-shared-gc flag, no extra code is compiled and so existing behaviour is maintained without any performance penalty.

Enabling this feature, but not populating RUBY_GC_LIBRARY_PATH will incur a small performance penalty due to the overhead of using getenv to check for the environment variable.

The use of dln.c rather than using dlopen directly, should enable this feature to be supported on all platforms that support loading shared libraries, rather than the initial implementation, which explicitly only supported Linux and MacOS.