kbuild: Use uname for LINUX_COMPILE_HOST detection

`hostname` may not be present on some systems as it's not mandated by
POSIX/SUSv4. This isn't just a theoretical problem: on Arch Linux,
`hostname` is provided by `inetutils`, which isn't part of the base
distribution.

    ./scripts/mkcompile_h: line 38: hostname: command not found

Use `uname -n` instead, which is more likely to be available (and
mandated by standards).

Signed-off-by: Chris Down <chris@chrisdown.name>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>