[Bug #19289] Retain `ruby_abi_version` function

	A few extension libraries, to hide all symbols except for necessary to
	load, hardcode the symbols to be exported in symbol list files for
	linker without even checking by `have_func`.  As a workaround for such
	libraries, retain `ruby_abi_version` symbol always even in released
	versions for now.
	---
	 include/ruby/internal/abi.h | 7 +++++--
	 1 file changed, 5 insertions(+), 2 deletions(-)

* See https://bugs.ruby-lang.org/issues/19078#note-30

The documentation says that the `end` pointer will be marked
but looking at the source, that is not the case.

Although the storage of the opaque Ruby object is an array of VALUEs, the intention of RB_ALLOCV is to allocate bytes of memory.

* Slightly more consistent indentation with other parts of the document.

Since https://github.com/ruby/ruby/pull/6599, RUBY_IMMEDIATE_MASK also
overlaps RUBY_Qnil. Now RB_SPECIAL_CONST_P seems confusing since both
RB_IMMEDIATE_P and RB_TEST check for RUBY_Qnil while we only need to
check RUBY_Qnil besides RUBY_IMMEDIATE_MASK. I'd like to make this
change to make it less confusing.

I confirmed that this doesn't change the number of instructions used for
the RUBY_Qfalse check on Linux x86_64 GCC and macOS arm64 Clang.

Since object shapes store the capacity of an object, we no longer
need the numiv field on RObjects. This gives us one extra slot which
we can use to give embedded objects one more instance variable (for a
total of 3 ivs). This commit removes the concept of numiv from RObject.