[ELF] Allow TargetLayout descendants to control assignment sections to segments

The TargetLayout class puts two sections into the same segment if they
have equal segment types and the same section flags (SHF_xxx). To be
able to merge some sort of sections into the same segment we drop some
flags before comparison. For example to merge string sections into Data
segment we drop SHF_STRINGS and SHF_MERGE flags.

The patch allows TargetLayout descendants to drop some target specific
section flags. MIPS target needs that to merge .MIPS.options section
which has SHF_MIPS_NOSTRIP flag into the LOAD segment.

http://reviews.llvm.org/D9160

llvm-svn: 235487