loongarch, crash: wrap crash dumping code into crash related ifdefs

Now crash codes under kernel/ folder has been split out from kexec
code, crash dumping can be separated from kexec reboot in config
items on loongarch with some adjustments.

Here use IS_ENABLED(CONFIG_CRASH_RESERVE) check to decide if compiling
in the crashkernel reservation code.

Link: https://lkml.kernel.org/r/20240124051254.67105-15-bhe@redhat.comSigned-off-by: Baoquan He <bhe@redhat.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Eric W. Biederman <ebiederm@xmission.com>
Cc: Hari Bathini <hbathini@linux.ibm.com>
Cc: Pingfan Liu <piliu@redhat.com>
Cc: Klara Modin <klarasmodin@gmail.com>
Cc: Michael Kelley <mhklinux@outlook.com>
Cc: Nathan Chancellor <nathan@kernel.org>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Yang Li <yang.lee@linux.alibaba.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

arch/loongarch/kernel/setup.c

@@ -260,7 +260,7 @@ static void __init arch_reserve_crashkernel(void)
 	char *cmdline = boot_command_line;
 	bool high = false;
 
-	if (!IS_ENABLED(CONFIG_KEXEC_CORE))
+	if (!IS_ENABLED(CONFIG_CRASH_RESERVE))
 		return;
 
 	ret = parse_crashkernel(cmdline, memblock_phys_mem_size(),