LoongArch: Add support for kernel address space layout randomization (KASLR)

LoongArch inclusion
category: feature
bugzilla: https://gitee.com/openeuler/kernel/issues/I736HO



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This patch adds support for relocating the kernel to a random address.

Entropy is derived from the banner, which will change every build and
random_get_entropy() which should provide additional runtime entropy.

The kernel is relocated by up to RANDOMIZE_BASE_MAX_OFFSET bytes from
its link address. Because relocation happens so early in the kernel boot,
the amount of physical memory has not yet been determined. This means
the only way to limit relocation within the available memory is via
Kconfig. Limit the maximum value of RANDOMIZE_BASE_MAX_OFFSET to
256M(0x10000000) because our memory layout has many holes.

KERNELOFFSET (kaslr_offset) is added to vmcoreinfo in the future, for
crash --kaslr support.

Signed-off-by: Youling Tang <tangyouling@loongson.cn>
Change-Id: I1e2e5cda0445e6f2c431638694fe822794b63442