[RuntimeDyld] Always allocate at least 1 byte for object sections in the JIT to

ensure that section addresses are distinct.

mapSectionAddress will fail if two sections are allocated the same address,
which can happen if any section has zero size (since malloc(0) is implementation
defined). Unfortunately I've been unable to repro this with a simple test case.

Fixes <rdar://problem/20314015>.

llvm-svn: 234299