Pin more references and expose some globally rooted symbols

[qinsoon]

This PR adds a few more pining for types in the native heap, and exposes globally rooted symbols so we can trace them in the MMTk binding. With this PR, we can run moving Immix without transitively pinning global_roots_table. See mmtk/mmtk-julia#170.

• Add jl_gc_pin
• Use jl_gc_pin to pin BigFloat for MPFR
• Rename PTR_PIN to OBJ_PIN. Additionally add PTR_PIN which handles cases for internal pointers.
• Pin objects that are used as key in jl_codectx_t.global_targets
• Pin objects that are stored in jl_codectx_t.PhiNodes
• Pin objects that are referred to by jl_cgval_t
• Disable GC before doing perm alloc in jl_get_layout (see Julia assumes perm alloc won't trigger GC mmtk-julia#172)
• Pin weak references and the referee (see 'Called is_live() on 0, which maps to an empty space' in weak ref processing mmtk-julia#176)
• Pin objects that are used in libuv's handle.
• Pin jl_task_t.completion_future (see unlock count must match lock count mmtk-julia#179)
• Pin all jl_codeinst_t objects.
• Pin all jl_method_instance_t objects.
• Pin all jl_module_t objects.
• Pin all jl_task_t objects.
• Expose some JL_GLOBALLY_ROOTED symbols, such as newly_inferred, task_done_hook_func.

[udesou]

LGTM.