Don't include PHDRs if linker script doesn't want them

This script below shouldn't include file and program headers
to PT_LOAD segment, because it doesn't have PHDRS and FILEHDR
attributes:

PHDRS { all PT_LOAD; }
SECTIONS { /* list of sections here */ }

Differential revision: https://reviews.llvm.org/D25774

llvm-svn: 284709