RFC: Remove output_target in ld

[H. J. Lu]

ld has both output_target and ld_emulation, which has a target_name
field. Both of them are supposed to specify the output target name.
However, they aren't consistent, especially when a linker supports
different endians. Why do we need output_target? Can we set
ld_emulation correctly and use ld_emulation->target_name to replace
output_target. Also ld supports the GNUTARGET enviroment variable.
But we don't use it to set ld_emulation. I'd like to modify ld to
make ld_emulation consistent:

1. Initialize ld_emulation properly with
   a. GNUTARGET.
   b. -m EMULATION.
   and correct endian, based on -EL/-BL.
2. Use ld_emulation->target_name to identify the output target.


H.J.