[PATCH] [Asan] Pack signal context into a structure

[Alexey Samsonov]

LGTM (but please the problem for powerpc before commit)

================
Comment at: lib/asan/asan_posix.cc:61
@@ -55,3 +60,3 @@
   // below sp.
   if (!IsStackAccess && IsAccessibleMemoryRange(pc, 4)) {
     u32 inst = *(unsigned *)pc;
----------------
Please fix the code under this #if:  s/pc/sig.pc.

http://reviews.llvm.org/D6148