crash_dump: Make elfcorehdr address/size symbols always visible

Make the forward declarations of elfcorehdr_addr and elfcorehdr_size,
and the definitions of ELFCORE_ADDR_MAX and ELFCORE_ADDR_ERR always
available, like is done for phys_initrd_start and phys_initrd_size.
Code referring to these symbols can then just check for
IS_ENABLED(CONFIG_CRASH_DUMP), instead of requiring conditional
compilation using an #ifdef, thus preparing to increase compile
coverage.

Suggested-by: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/ba965ca613c0cc82c1ec2fe353ee34fb13b36474.1628670468.git.geert+renesas@glider.be

Author: geertu

include/linux/crash_dump.h

@@ -10,13 +10,14 @@
 
 #include <linux/pgtable.h> /* for pgprot_t */
 
-#ifdef CONFIG_CRASH_DUMP
+/* For IS_ENABLED(CONFIG_CRASH_DUMP) */
 #define ELFCORE_ADDR_MAX	(-1ULL)
 #define ELFCORE_ADDR_ERR	(-2ULL)
 
 extern unsigned long long elfcorehdr_addr;
 extern unsigned long long elfcorehdr_size;
 
+#ifdef CONFIG_CRASH_DUMP
 extern int elfcorehdr_alloc(unsigned long long *addr, unsigned long long *size);
 extern void elfcorehdr_free(unsigned long long addr);
 extern ssize_t elfcorehdr_read(char *buf, size_t count, u64 *ppos);