Slapping __percpu_qual into the next available header is sloppy at best.

It's required by __percpu which is defined in compiler_types.h and that is
meant to be included without requiring a boatload of other headers so that
a struct or function declaration can contain a __percpu qualifier w/o
further prerequisites.

This implicit dependency on linux/percpu.h makes that impossible and causes
a major problem when trying to separate headers.

Create asm/percpu_types.h and move it there. Include that from
compiler_types.h and the whole recursion problem goes away.

Fix up UM so it uses the generic header and includes it in the UM_HOST
build, which pulls in compiler_types.h. The USER_CFLAGS fix was suggested
by Richard.

Signed-off-by: Thomas Gleixner <tglx@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://patch.msgid.link/20260602090535.254874125@kernel.org