[sanitizer] Allow Fuchsia symbolizer to be reused by Myriad RTEMS

Like Fuchsia, Myriad RTEMS uses an off-line symbolizer -- we just need
a custom backtrace frame string.  Move this definition to
sanitizer_fuchsia.h; the corresponding RTEMS one will be added when we
add sanitizer_rtems.h.

Differential Revision: https://reviews.llvm.org/D46462

llvm-svn: 332157