[asan] Port asan_malloc_linux.cc to RTEMS

We reuse the allocation interceptors as is.  RTEMS doesn't support
dlsyms.  However, it needs to handle memory allocation requests before
the ASan run-time has been initialized.  We use the dlsym alloc pool
for this purpose, and we increase its size to 4k to support this
usage.

Differential Revision: https://reviews.llvm.org/D46465

llvm-svn: 331649