makefile: Add `norelro` linker option

RELRO is an instruction to a dynamic loader to make a memory range
read-only after relocations are applied, for added security. Some
linkers (e.g. LLD) require that all sections covered by the RELRO are
contiguous, so that only a single RELRO is needed. U-Boot at present
neither satisfies this requirement (e.g. x86_64 linker script currently
puts .dynamic too far from .got) nor preserves the RELRO when converting
away from ELF, therefore add `-z norelro` to global linker options.

This can be brought back in the future when the linker scripts are
cleaned up and U-Boot understands RELROs.

Signed-off-by: Sam Edwards <CFSworks@gmail.com>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>

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