um: Allow builds with Clang

Add SUBARCH target for Clang+um (which must go last, not alphabetically,
so the other SUBARCHes are assigned). Remove open-coded "DEFINE"
macro, instead using linux/kbuild.h's version which was updated to use
Clang-friendly assembly in commit cf0c3e68aa81 ("kbuild: fix asm-offset
generation to work with clang"). Redefine "DEFINE_LONGS" in terms of
"COMMENT" and "DEFINE" so that the intended coment actually has useful
content. Add a missed "break" to avoid implicit fall-through warnings.

This lets me run KUnit tests with Clang:

$ ./tools/testing/kunit/kunit.py run --make_options LLVM=1
...

Cc: Jeff Dike <[email protected]>
Cc: Richard Weinberger <[email protected]>
Cc: Anton Ivanov <[email protected]>
Cc: Masahiro Yamada <[email protected]>
Cc: Nick Desaulniers <[email protected]>
Cc: Nathan Chancellor <[email protected]>
Cc: David Gow <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Reviewed-by: Nathan Chancellor <[email protected]>
Link: https://lore.kernel.org/lkml/Yg2YubZxvYvx7%[email protected]/
Tested-by: David Gow <[email protected]>
Link: https://lore.kernel.org/lkml/CABVgOSk=oFxsbSbQE-v65VwR2+mXeGXDDjzq8t7FShwjJ3+kUg@mail.gmail.com/
Signed-off-by: Kees Cook <[email protected]>
---
v1: https://lore.kernel.org/lkml/[email protected]
v2: https://lore.kernel.org/lkml/[email protected]
v3:
 - use kbuild.h to avoid duplication (Masahiro)
 - fix intended comments (Masahiro)
 - use SUBARCH (Nathan)

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