Introduce __read_frequently

While __read_mostly groups variables together, their placement is not
specified. In particular 2 frequently used variables can end up in
different lines.

This annotation is only expected to be used for variables read all the time,
e.g. on each syscall entry.

MFC after:	1 week

sys/conf/ldscript.amd64

@@ -146,6 +146,10 @@ SECTIONS
   . = DATA_SEGMENT_RELRO_END (24, .);
   .got.plt        : { *(.got.plt) }
   . = ALIGN(64);
+  .data.read_frequently :
+  {
+    *(.data.read_frequently)
+  }
   .data.read_mostly :
   {
     *(.data.read_mostly)

sys/sys/systm.h

@@ -138,6 +138,7 @@ void	kassert_panic(const char *fmt, ...)  __printflike(1, 2);
  * Align variables.
  */
 #define	__read_mostly		__section(".data.read_mostly")
+#define	__read_frequently	__section(".data.read_frequently")
 #define	__exclusive_cache_line	__aligned(CACHE_LINE_SIZE) \
 				    __section(".data.exclusive_cache_line")
 /*