kbuild: fix SIGPIPE error message for AR=gcc-ar and AR=llvm-ar

kbuild: fix SIGPIPE error message for AR=gcc-ar and AR=llvm-ar

Jiri Slaby reported that building the kernel with AR=gcc-ar shows:
  /usr/bin/ar terminated with signal 13 [Broken pipe]

Nathan Chancellor reported the latest AR=llvm-ar shows:
  error: write on a pipe with no reader

The latter occurs since LLVM commit 51b557adc131 ("Add an error message
to the default SIGPIPE handler").

The resulting vmlinux is correct, but it is better to silence it.

'head -n1' exits after reading the first line, so the pipe is closed.

Use 'sed -n 1p' to eat the stream till the end.

Fixes: 40aeb3ea8e96 ("kbuild: use obj-y instead extra-y for objects placed at the head")
Link: https://github.com/ClangBuiltLinux/linux/issues/1651
Reported-by: Jiri Slaby <jirislaby@kernel.org>
Reported-by: Nathan Chancellor <nathan@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Tested-by: Nick Desaulniers <ndesaulniers@google.com>
Reviewed-by: Nick Desaulniers <ndesaulniers@google.com>
Tested-by: Nathan Chancellor <nathan@kernel.org>