[sanitizer] First step toward supporting 42-bit AS on aarch64

[Jakub Jelinek]

Hi!

aarch64-linux kernel has configurable 39, 42 or 47 bit virtual address
space.  Most distros AFAIK use 42-bit VA right now, but there are also
39-bit VA users too.  The ppc64 handling can be used for this just fine
and support all the 3 sizes.

There are other issues, like allocator32 not really being able to support
the larger addres spaces, and hardcoded 39-bit address space size in other
macros.

--- compiler-rt/lib/sanitizer_common/sanitizer_posix.cc.jj	2014-12-15 19:50:53.618624013 +0100
+++ compiler-rt/lib/sanitizer_common/sanitizer_posix.cc	2015-01-20 18:53:21.012780056 +0100
@@ -78,16 +78,15 @@ static uptr GetKernelAreaSize() {
 
 uptr GetMaxVirtualAddress() {
 #if SANITIZER_WORDSIZE == 64
-# if defined(__powerpc64__)
+# if defined(__powerpc64__) || defined(__aarch64__)
   // On PowerPC64 we have two different address space layouts: 44- and 46-bit.
   // We somehow need to figure out which one we are using now and choose
   // one of 0x00000fffffffffffUL and 0x00003fffffffffffUL.
   // Note that with 'ulimit -s unlimited' the stack is moved away from the top
   // of the address space, so simply checking the stack address is not enough.
   // This should (does) work for both PowerPC64 Endian modes.
+  // Similarly, aarch64 has multiple address space layouts: 39, 42 and 47-bit.
   return (1ULL << (MostSignificantSetBitIndex(GET_CURRENT_FRAME()) + 1)) - 1;
-# elif defined(__aarch64__)
-  return (1ULL << 39) - 1;
 # elif defined(__mips64)
   return (1ULL << 40) - 1;  // 0x000000ffffffffffUL;
 # else

	Jakub