x86: kdump: use generic interface to simplify crashkernel reservation code

With the help of newly changed function parse_crashkernel() and generic
reserve_crashkernel_generic(), crashkernel reservation can be simplified
by steps:

1) Add a new header file <asm/crash_core.h>, and define CRASH_ALIGN,
   CRASH_ADDR_LOW_MAX, CRASH_ADDR_HIGH_MAX and
   DEFAULT_CRASH_KERNEL_LOW_SIZE in <asm/crash_core.h>;

2) Add arch_reserve_crashkernel() to call parse_crashkernel() and
   reserve_crashkernel_generic(), and do the ARCH specific work if
   needed.

3) Add ARCH_HAS_GENERIC_CRASHKERNEL_RESERVATION Kconfig in
   arch/x86/Kconfig.

When adding DEFAULT_CRASH_KERNEL_LOW_SIZE, add crash_low_size_default() to
calculate crashkernel low memory because x86_64 has special requirement.

The old reserve_crashkernel_low() and reserve_crashkernel() can be
removed.

[bhe@redhat.com: move crash_low_size_default() code into <asm/crash_core.h>]
  Link: https://lkml.kernel.org/r/ZQpeAjOmuMJBFw1/@MiWiFi-R3L-srv
Link: https://lkml.kernel.org/r/20230914033142.676708-7-bhe@redhat.com


Signed-off-by: Baoquan He <bhe@redhat.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Chen Jiahao <chenjiahao16@huawei.com>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>