vmlinux.lds.h: place optional header space in BOUNDED_SECTION

Extend recently added BOUNDED_SECTION(_name) macro by adding a
KEEP(*(.gnu.linkonce.##_name)) before the KEEP(*(_name)).

This does nothing by itself, vmlinux is the same before and after this
patch.  But if a developer adds a .gnu.linkonce.foo record, that
record is placed in the front of the section, where it can be used as
a header for the table.

The intent is to create an up-link to another organizing struct, from
where related tables can be referenced.  And since every item in a
table has a known offset from its header, that same offset can be used
to fetch records from the related tables.

By itself, this doesnt gain much, unless maybe the pattern of access
is to scan 1 or 2 fields in each fat record, but with 2 16 bit .map*
fields added, we could de-duplicate 2 related tables.

The use case here is struct _ddebug, which has 3 pointers (function,
file, module) with substantial repetition; respectively 53%, 90%, and
the module column is fully recoverable after dynamic_debug_init()
splits the table into a linked list of "module" chunks.

On a DYNAMIC_DEBUG=y kernel with 5k pr_debugs, the memory savings
should be ~100 KiB.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
Link: https://lore.kernel.org/r/20221022225637.1406715-3-jim.cromie@gmail.com
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

include/asm-generic/vmlinux.lds.h

@@ -195,11 +195,13 @@
 
 #define BOUNDED_SECTION_PRE_LABEL(_sec_, _label_, _s_, _e_)		\
 	_s_##_label_ = .;						\
+	KEEP(*(.gnu.linkonce.##_sec_))					\
 	KEEP(*(_sec_))							\
 	_e_##_label_ = .;
 
 #define BOUNDED_SECTION_POST_LABEL(_sec_, _label_, _s_, _e_)		\
 	_label_##_s_ = .;						\
+	KEEP(*(.gnu.linkonce.##_sec_))					\
 	KEEP(*(_sec_))							\
 	_label_##_e_ = .;