Implement the JIT side of the GDB JIT debugging interface. To enable this

Implement the JIT side of the GDB JIT debugging interface.  To enable this
feature, either build the JIT in debug mode to enable it by default or pass
-jit-emit-debug to lli.

Right now, the only debug information that this communicates to GDB is call
frame information, since it's already being generated to support exceptions in
the JIT.  Eventually, when DWARF generation isn't tied so tightly to AsmPrinter,
it will be easy to push that information to GDB through this interface.

Here's a step-by-step breakdown of how the feature works:

- The JIT generates the machine code and DWARF call frame info
  (.eh_frame/.debug_frame) for a function into memory.
- The JIT copies that info into an in-memory ELF file with a symbol for the
  function.
- The JIT creates a code entry pointing to the ELF buffer and adds it to a
  linked list hanging off of a global descriptor at a special symbol that GDB
  knows about.
- The JIT calls a function marked noinline that GDB knows about and has put an
  internal breakpoint in.
- GDB catches the breakpoint and reads the global descriptor to look for new
  code.
- When sees there is new code, it reads the ELF from the inferior's memory and
  adds it to itself as an object file.
- The JIT continues, and the next time we stop the program, we are able to
  produce a proper backtrace.

Consider running the following program through the JIT:

#include <stdio.h>
void baz(short z) {
  long w = z + 1;
  printf("%d, %x\n", w, *((int*)NULL));  // SEGFAULT here
}
void bar(short y) {
  int z = y + 1;
  baz(z);
}
void foo(char x) {
  short y = x + 1;
  bar(y);
}
int main(int argc, char** argv) {
  char x = 1;
  foo(x);
}

Here is a backtrace before this patch:
Program received signal SIGSEGV, Segmentation fault.
[Switching to Thread 0x2aaaabdfbd10 (LWP 25476)]
0x00002aaaabe7d1a8 in ?? ()
(gdb) bt
#0  0x00002aaaabe7d1a8 in ?? ()
#1  0x0000000000000003 in ?? ()
#2  0x0000000000000004 in ?? ()
#3  0x00032aaaabe7cfd0 in ?? ()
#4  0x00002aaaabe7d12c in ?? ()
#5  0x00022aaa00000003 in ?? ()
#6  0x00002aaaabe7d0aa in ?? ()
#7  0x01000002abe7cff0 in ?? ()
#8  0x00002aaaabe7d02c in ?? ()
#9  0x0100000000000001 in ?? ()
#10 0x00000000014388e0 in ?? ()
#11 0x00007fff00000001 in ?? ()
#12 0x0000000000b870a2 in llvm::JIT::runFunction (this=0x1405b70,
F=0x14024e0, ArgValues=@0x7fffffffe050)
   at /home/rnk/llvm-gdb/lib/ExecutionEngine/JIT/JIT.cpp:395
#13 0x0000000000baa4c5 in llvm::ExecutionEngine::runFunctionAsMain
(this=0x1405b70, Fn=0x14024e0, argv=@0x13f06f8, envp=0x7fffffffe3b0)
   at /home/rnk/llvm-gdb/lib/ExecutionEngine/ExecutionEngine.cpp:377
#14 0x00000000007ebd52 in main (argc=2, argv=0x7fffffffe398,
envp=0x7fffffffe3b0) at /home/rnk/llvm-gdb/tools/lli/lli.cpp:208

And a backtrace after this patch:
Program received signal SIGSEGV, Segmentation fault.
0x00002aaaabe7d1a8 in baz ()
(gdb) bt
#0  0x00002aaaabe7d1a8 in baz ()
#1  0x00002aaaabe7d12c in bar ()
#2  0x00002aaaabe7d0aa in foo ()
#3  0x00002aaaabe7d02c in main ()
#4  0x0000000000b870a2 in llvm::JIT::runFunction (this=0x1405b70,
F=0x14024e0, ArgValues=...)
   at /home/rnk/llvm-gdb/lib/ExecutionEngine/JIT/JIT.cpp:395
#5  0x0000000000baa4c5 in llvm::ExecutionEngine::runFunctionAsMain
(this=0x1405b70, Fn=0x14024e0, argv=..., envp=0x7fffffffe3c0)
   at /home/rnk/llvm-gdb/lib/ExecutionEngine/ExecutionEngine.cpp:377
#6  0x00000000007ebd52 in main (argc=2, argv=0x7fffffffe3a8,
envp=0x7fffffffe3c0) at /home/rnk/llvm-gdb/tools/lli/lli.cpp:208

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@82418 91177308-0d34-0410-b5e6-96231b3b80d8