[ELF] - Implemented -znotext

gold linker manual describes them as:

-z text	Do not permit relocations in read-only segments
-z notext Permit relocations in read-only segments (default)

In LLD default is to not permit them. Patch implements -z notext.

Differential revision: https://reviews.llvm.org/D30530

llvm-svn: 297366