nios2: linker.ld: ensure reset code isn't discarded

This isn't directly referenced by other code in the binary,
it just sits at a magic memory address. Make sure gc-sections
doesn't throw it away.

Change-Id: I1c00a163dbf2eb4866ebadc7f1d70bcc6845b8d1
Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>