port core.sys.linux.elf

Based on [this](http://sourceware.org/git/?p=glibc.git;a=blob;f=elf/elf.h;h=9bc5004c1724e8d93d04d09ec07ccf3d9baaf2ce;hb=HEAD).
I'm a little concerned about copyright implications. AFAIK constants and declarations are fine, but we might have to remove comments and rewrite the macro functions.