[ELF] Don't emit empty PT_LOAD segment

Sometimes the very first PT_LOAD segment, created by lld, can be empty. 
This happens when (all conditions met):

- Linker script is used
- First section in ELF image is not RO
- Not enough space for program headers.

Differential revision: https://reviews.llvm.org/D25330

llvm-svn: 283760