x86: kdump: move reserve_crashkernel_low() into kexec_core.c

hulk inclusion
category: feature
bugzilla: 12800
CVE: NA

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In preparation for supporting more than one crash kernel regions
in arm64 as x86_64 does, move reserve_crashkernel_low() into
kexec/kexec_core.c.

Signed-off-by: Chen Zhou <chenzhou10@huawei.com>
Reviewed-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>