fix some warnings emitted by 'ruby -w'

- File.exists? is deprecated since 2.1.0
- tasks/mruby_build_commands.rake:268: ambiguous argument
codegen.c: remove superfluous return
NaN boxing: use uintptr_t for bit manipulations
remove various preprocessor conditionals

- HAVE_IEEEFP_H is nowhere defined or needed at all
- FreeBSD < 4 is unsupported since years
- MSVC workaround (around what exactly?)
use the same error msg as mrb_str_to_cstr/CRuby

It's better to have one message for the same error
than two different (although similar worded) ones.
Fix extended arena check in gc_protect
Merge pull request #1631 from cremno/fix-ruby-warnings

fix some warnings emitted by 'ruby -w'
Merge pull request #1633 from cremno/nan-boxing-uintptr_t

NaN boxing: use uintptr_t for bit manipulations
Merge pull request #1635 from cremno/cpp-conditionals

remove various preprocessor conditionals
Merge pull request #1636 from cremno/mrb_get_args-z-errmsg

use the same error msg as mrb_str_to_cstr/CRuby
Merge pull request #1637 from h2so5/extended-arena-size

Fix extended arena check in gc_protect
Merge pull request #1632 from cremno/superfluous-return

remove superfluous return in codedump_all