runtime constants: deal with old decrepit linkers

The runtime constants linker script depended on documented linker
behavior [1]:

 "If an output section’s name is the same as the input section’s name
  and is representable as a C identifier, then the linker will
  automatically PROVIDE two symbols: __start_SECNAME and __stop_SECNAME,
  where SECNAME is the name of the section. These indicate the start
  address and end address of the output section respectively"

to just automatically define the symbol names for the bounds of the
runtime constant arrays.

It turns out that this isn't actually something we can rely on, with old
linkers not generating these automatic symbols.  It looks to have been
introduced in binutils-2.29 back in 2017, and we still support building
with versions all the way back to binutils-2.25 (from 2015).

And yes, Oleg actually seems to be using such ancient versions of
binutils.

So instead of depending on the implicit symbols from "section names
match and are representable C identifiers", just do this all manually.
It's not like it causes us any extra pain, we already have to do that
for all the other sections that we use that often have special
characters in them.
Reported-and-tested-by: Oleg Nesterov <oleg@redhat.com>
Link: https://sourceware.org/binutils/docs/ld/Input-Section-Example.html [1]
Link: https://lore.kernel.org/all/20240802114518.GA20924@redhat.com/Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

include/asm-generic/vmlinux.lds.h

@@ -911,13 +911,12 @@
 #define CON_INITCALL							\
 	BOUNDED_SECTION_POST_LABEL(.con_initcall.init, __con_initcall, _start, _end)
 
-#define RUNTIME_NAME(t,x) runtime_##t##_##x
+#define NAMED_SECTION(name) \
+	. = ALIGN(8); \
+	name : AT(ADDR(name) - LOAD_OFFSET) \
+	{ BOUNDED_SECTION_PRE_LABEL(name, name, __start_, __stop_) }
 
-#define RUNTIME_CONST(t,x)						\
-	. = ALIGN(8);							\
-	RUNTIME_NAME(t,x) : AT(ADDR(RUNTIME_NAME(t,x)) - LOAD_OFFSET) {	\
-		*(RUNTIME_NAME(t,x));					\
-	}
+#define RUNTIME_CONST(t,x) NAMED_SECTION(runtime_##t##_##x)
 
 /* Alignment must be consistent with (kunit_suite *) in include/kunit/test.h */
 #define KUNIT_TABLE()							\