[sanitizer] Replace a local array with InternalScopedString in MaybeReexec()

`MaybeReexec` contains a 1024-byte long local array, which produces a warning about frame size:

   .../lib/sanitizer_common/sanitizer_mac.cc:548:6: warning: stack frame size of 1132 bytes in function '__sanitizer::MaybeReexec' [-Wframe-larger-than=]

Let's replace it with InternalScopedString.

Differential Revision: http://reviews.llvm.org/D15181

llvm-svn: 254619