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


Differential Revision:

git-svn-id: 91177308-0d34-0410-b5e6-96231b3b80d8
diff --git a/src/DwarfParser.hpp b/src/DwarfParser.hpp
index 26993c4..a19f051 100644
--- a/src/DwarfParser.hpp
+++ b/src/DwarfParser.hpp
@@ -380,7 +380,9 @@
     uint64_t length;
     uint8_t opcode = addressSpace.get8(p);
     uint8_t operand;
+#if !defined(_LIBUNWIND_NO_HEAP)
     PrologInfoStackEntry *entry;
     switch (opcode) {
     case DW_CFA_nop:
@@ -492,6 +494,7 @@
         fprintf(stderr, "DW_CFA_register(reg=%" PRIu64 ", reg2=%" PRIu64 ")\n",
                 reg, reg2);
+#if !defined(_LIBUNWIND_NO_HEAP)
     case DW_CFA_remember_state:
       // avoid operator new, because that would be an upward dependency
       entry = (PrologInfoStackEntry *)malloc(sizeof(PrologInfoStackEntry));
@@ -517,6 +520,7 @@
       if (logDwarf)
         fprintf(stderr, "DW_CFA_restore_state\n");
     case DW_CFA_def_cfa:
       reg = addressSpace.getULEB128(p, instructionsEnd);
       offset = (int64_t)addressSpace.getULEB128(p, instructionsEnd);
diff --git a/src/UnwindCursor.hpp b/src/UnwindCursor.hpp
index 040d13e..9068316 100644
--- a/src/UnwindCursor.hpp
+++ b/src/UnwindCursor.hpp
@@ -114,6 +114,7 @@
 template <typename A>
 void DwarfFDECache<A>::add(pint_t mh, pint_t ip_start, pint_t ip_end,
                            pint_t fde) {
+#if !defined(_LIBUNWIND_NO_HEAP)
   if (_bufferUsed >= _bufferEnd) {
     size_t oldSize = (size_t)(_bufferEnd - _buffer);
@@ -139,6 +140,7 @@
 template <typename A>