kbuild: rpm-pkg: Fix C locale setup

semicolon separation in LC_ALL is wrong. Either variable needs to be
exported before as a separate commit or set as part of the commit in the
beginning. Used second variant.

This fixes broken build on user's locale setup which makes 'date' binary
to produce invalid characters in rpm changelog (e.g. cs_CZ.UTF-8 'čec'):

$ make binrpm-pkg
  GEN     rpmbuild/SPECS/kernel.spec
rpmbuild -bb rpmbuild/SPECS/kernel.spec --define='_topdirlinux/rpmbuild' \
    --target x86_64-linux --build-in-place --noprep --define='_smp_mflags \
    %{nil}' $(rpm -q rpm >/dev/null 2>&1 || echo --nodeps)
Building target platforms: x86_64-linux
Building for target x86_64-linux
error: bad date in %changelog: St čec 24 2024 user <user@somehost>
make[2]: *** [scripts/Makefile.package:71: binrpm-pkg] Error 1
make[1]: *** [linux/Makefile:1546: binrpm-pkg] Error 2
make: *** [Makefile:224: __sub-make] Error 2

Fixes: 301c1090

 ("kbuild: rpm-pkg: introduce a simple changelog section for kernel.spec")
Signed-off-by: Petr Vorel <pvorel@suse.cz>
Reviewed-by: Miguel Ojeda <ojeda@kernel.org>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>