[Driver] Use .init_array for all gcc installations and simplify Generic_ELF...

[Driver] Use .init_array for all gcc installations and simplify Generic_ELF -fno-use-init-array rules

D39317 made clang use .init_array when no gcc installations is found.
This change changes all gcc installations to use .init_array .

GCC 4.7 by default stopped providing .ctors/.dtors compatible crt files,
and stopped emitting .ctors for __attribute__((constructor)).
.init_array should always work.

FreeBSD rules are moved to FreeBSD.cpp to make Generic_ELF rules clean.

Reviewed By: rnk

Differential Revision: https://reviews.llvm.org/D71434