Kbuild updates for v5.3

- remove headers_{install,check}_all targets

- remove unreasonable 'depends on !UML' from CONFIG_SAMPLES

- re-implement 'make headers_install' more cleanly

- add new header-test-y syntax to compile-test headers

- compile-test exported headers to ensure they are compilable in
  user-space

- compile-test headers under include/ to ensure they are self-contained

- remove -Waggregate-return, -Wno-uninitialized, -Wno-unused-value flags

- add -Werror=unknown-warning-option for Clang

- add 128-bit built-in types support to genksyms

- fix missed rebuild of modules.builtin

- propagate 'No space left on device' error in fixdep to Make

- allow Clang to use its integrated assembler

- improve some coccinelle scripts

- add a new flag KBUILD_ABS_SRCTREE to request Kbuild to use absolute
  path for $(srctree).

- do not ignore errors when compression utility is missing

- misc cleanups
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