RFA: ELF: Call check_relocs after opening all inputs

[Joseph Myers]

This patch results in glibc failing to build for microblaze, and libstdc++ 
failing to build for tilegx / tilepro.  These are all BFD internal errors 
in the linker.  Those configurations all build OK with the previous 
binutils commit and fail with the commit of this patch.

https://sourceware.org/ml/libc-testresults/2017-q4/msg00113.html

build-many-glibcs.py ("compilers" task for microblaze-linux-gnu 
tilegx-linux-gnu tilepro-linux-gnu) should reproduce those failures when 
you use mainline binutils and GCC (mainline GCC probably not needed, 
that's just the combination I tested).  I don't have reduced tests.

-- 
Joseph S. Myers
joseph@codesourcery.com