x86/vmlinux: Drop unneeded linker script discard of .eh_frame

Now that .eh_frame sections for the files in setup.elf and realmode.elf
are not generated anymore, the linker scripts don't need the special
output section name /DISCARD/ any more.

Remove the one in the main kernel linker script as well, since there are
no .eh_frame sections already, and fix up a comment referencing .eh_frame.

Update the comment in asm/dwarf2.h referring to .eh_frame so it continues
to make sense, as well as being more specific.

 [ bp: Touch up commit message. ]

Signed-off-by: Arvind Sankar <nivedita@alum.mit.edu>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Nathan Chancellor <natechancellor@gmail.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Kees Cook <keescook@chromium.org>
Tested-by: Nathan Chancellor <natechancellor@gmail.com>
Link: https://lkml.kernel.org/r/20200224232129.597160-3-nivedita@alum.mit.edu