Merge gc.h and internal/gc.h

[eightbitraptor]

History:

Currently we have 4 headers named gc.h

• gc.h
• internal/gc.h
• include/ruby/internal/gc.h
• include/ruby/internal/intern/gc.h

The first two are private and internal to the CRuby codebase, the latter 2 are visible to C extensions. This PR attempts to merge the two internal headers gc.h and internal/gc.hinto a single headerinternal/gc.h` in order to simplify the codebase and put all the gc related interface in a single place.

Here is the history of the 2 internal only gc.h headers as I've been able to understand from the commit logs:

• gc.h was created as part of the YARV merge in 2006, it contained only some defines for debug related macros.
• In 2010 some function declarations were added to it to expose some functionality (rb_objspace_each_objects) to the objspace extension
• In May 2011 internal.h was created as a place to put shared, but internal only declarations.
• In June 2011 GC related functions started being committed to internal.h
• In 2019 internal.h was refactored and internal/gc.h was created

As of 2023 internally used code continues to be added to both gc.h and internal/gc.h, which one is chosen in any particular circumstance looks to be whichever header is already being included for legacy reasons in the .c file being edited.

[peterzhu2118]

There's several places in depend files that have $(top_srcdir)/gc.h, I think these can also go away.