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/[e-mail]
Signed-off-by: Baoquan He <[e-mail]>
Cc: Al Viro <[e-mail]>
Cc: Eric W. Biederman <[e-mail]>
Cc: Hari Bathini <[e-mail]>
Cc: Pingfan Liu <[e-mail]>
Cc: Klara Modin <[e-mail]>
Cc: Michael Kelley <[e-mail]>
Cc: Nathan Chancellor <[e-mail]>
Cc: Stephen Rothwell <[e-mail]>
Cc: Yang Li <[e-mail]>
Signed-off-by: Andrew Morton <[e-mail]>

arch/loongarch/kernel/setup.c

···
 	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(),