MIPS: ES6 symbols: turn symbols into a proper primitive type

Port r14051 (e9140a92)

Original commit message:
(qua last week's TC39)

Specifically:
- Install Symbol constructor function on the global object.
- Adjust code generation for typeof.
- Remove IsSymbol built-in, IS_SYMBOL macro now defined using typeof.
- Remove hack that allowed symbols as constructor results, and some other special cases.
- Remove symbol_delegate and GetDelegate function.
- Extend ToBoolean stub to handle symbols.
- Extend ToNumber to return NaN on symbols.
- Poison symbol's toString function, and thereby ToString on symbols.

BUG=

Review URL: https://codereview.chromium.org/13007003

git-svn-id: https://v8.googlecode.com/svn/branches/bleeding_edge@14058 ce2b1a6d-e550-0410-aec6-3dcde31c8c00