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[ Upstream commit 589834b3 ]

In commit ebcc5928 ("arm64: Silence gcc warnings about arch ABI
drift"), the arm64 Makefile added -Wno-psabi to KBUILD_CFLAGS, which is
a GCC only option so clang rightfully complains:

warning: unknown warning option '-Wno-psabi' [-Wunknown-warning-option]

https://clang.llvm.org/docs/DiagnosticsReference.html#wunknown-warning-option

However, by default, this is merely a warning so the build happily goes
on with a slew of these warnings in the process.

Commit c3f0d0bc ("kbuild, LLVMLinux: Add -Werror to cc-option to
support clang") worked around this behavior in cc-option by adding
-Werror so that unknown flags cause an error. However, this all happens
silently and when an unknown flag is added to the build unconditionally
like -Wno-psabi, cc-option will always fail because there is always an
unknown flag in the list of flags. This manifested as link time failures
in the arm64...