binutils: disambiguate ARM targets

https://github.com/NixOS/nixpkgs/pull/30484#issuecomment-345472766

Since [1] libbfd is compiled with support for all available targets. However, it
can not choose whether an ARM ELF file is elf32-littlearm,
elf32-littlearm-symbian, or elf32-littlearm-vxworks, and fails with the "File
format is ambiguous" error.  Here [2] Alan Modra intended to prioritize the
first of the three, but although his patch was merged and reportedly solved the
issue, currently glibc 2.28.1 and 2.29.1 again fail to disambiguate these
targets.  This commit makes it prioritize elf32-littlearm over the other two.

[1] f8741c38
[2] https://sourceware.org/ml/binutils/2013-05/msg00271.html