Kbuild: enable TRIM_UNUSED_KSYMS again, with some guarding

In commit 5cf0fd591f2e ("Kbuild: disable TRIM_UNUSED_KSYMS option") I
disabled this option because it's hugely expensive at build time, and I
questioned how much use it gets.

Several people piped up and convinced me it's actually useful, so
instead of disabling it entirely, it now depends on EXPERT and gets
disabled by COMPILE_TEST builds so that 'allmodconfig' style things
don't enable it.

I still hope somebody will take a look at the build time issue, because
as Arnd also noted:

"However, the combination of thinlto and trim indeed has a steep cost
in compile time, taking almost twice as long as a normal defconfig
(gc-sections makes it slightly faster)"

Cc: Masahiro Yamada <[e-mail]>
Cc: Arnd Bergmann <[e-mail]>
Cc: Jessica Yu <[e-mail]>
Cc: Cristoph Hellwig <[e-mail]>,
Cc: Miroslav Benes <[e-mail]>
Cc: Emil Velikov <[e-mail]>
Signed-off-by: Linus Torvalds <[e-mail]>

init/Kconfig

···
 	  If unsure, say N.
 
 config TRIM_UNUSED_KSYMS
-	bool "Trim unused exported kernel symbols"
-	depends on BROKEN
+	bool "Trim unused exported kernel symbols" if EXPERT
+	depends on !COMPILE_TEST
 	help
 	  The kernel and some modules make many symbols available for
 	  other modules to use via EXPORT_SYMBOL() and variants. Depending