Do not use private linkage for slice literal buffers

[HertzDevil]

The following snippet used to fail because a method in Foo's LLVM module refers to a $Slice:* buffer variable defined in the main LLVM module:

module Foo
  def self.foo
    Slice.literal(1)
  end
end

Foo.foo

The correct linkage is now used for the $Slice:* variables.

The existing workaround is to move the literal to a constant, in which case the constant initializer (const_init or const_read) is also defined in the main LLVM module, thus eliminating the link-time failure. But see also #15701.