[XRay][compiler-rt] FDR Mode: Use mmap instead of internal allocator

Summary:
This change moves FDR mode to use `internal_mmap(...)` from
sanitizer_common instead of the internal allocator interface. We're
doing this to sidestep the alignment issues we encounter with the
`InternalAlloc(...)` functions returning pointers that have some magic
bytes at the beginning.

XRay copies bytes into the buffer memory, and does not require the magic
bytes tracking the other sanitizers use when allocating/deallocating
buffers.

Reviewers: kpw, eizan

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D49972

llvm-svn: 338228