Make it possible to use libunwind without heap.

This patch allows to use libunwind on bare-metal systems that do not
include malloc/free by conditionally turning off nonessential
functionality that requires these functions.

The disabled functionality includes:

  * the .cfi_remember_state and .cfi_restore_state instructions;
  * the DWARF FDE cache.

The .cfi_{remember,restore}_state instructions don't seem to be used
by contemporary compilers. None of the LLVM backends emit it.

The DWARF FDE cache is bypassed if _LIBUNWIND_NO_HEAP is defined.
Specifically, entries are never added to it, so the search begins
and ends at the statically allocated, empty initial cache.

Such heap-less libunwind on a bare metal system is successfully used
in the ARTIQ project[1], and it is my hope that it will be useful
elsewhere.

[1]: http://m-labs.hk/artiq

Differential Revision: http://reviews.llvm.org/D11897

llvm-svn: 252452