Fix crossbuild bootstrap tools build with Clang 12

Clang 12 no longer allows re-defining a weak symbol as non-weak. This
happed here because we compile err.c with _err defined to err. To fix
this, u

Fix crossbuild bootstrap tools build with Clang 12

Clang 12 no longer allows re-defining a weak symbol as non-weak. This
happed here because we compile err.c with _err defined to err. To fix
this, use the same approach as the libc namespace.h

(cherry picked from commit 02af91c52e71e8a0f47251e637c9687f35d45dd9)

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