h8300: use cc-cross-prefix instead of hardcoding h8300-unknown-linux-

It believe it is a bad idea to hardcode a specific compiler prefix
that may or may not be installed on a user's system. It is annoying
when testing features that should not require compilers at all.

For example, mrproper, headers_install, etc. should work without
any compiler.

They look like follows on my machine.

$ make ARCH=h8300 mrproper
./scripts/gcc-version.sh: line 26: h8300-unknown-linux-gcc: command not found
./scripts/gcc-version.sh: line 27: h8300-unknown-linux-gcc: command not found
make: h8300-unknown-linux-gcc: Command not found
make: h8300-unknown-linux-gcc: Command not found
  [ a bunch of the same error messages continue ]

$ make ARCH=h8300 headers_install
./scripts/gcc-version.sh: line 26: h8300-unknown-linux-gcc: command not found
./scripts/gcc-version.sh: line 27: h8300-unknown-linux-gcc: command not found
make: h8300-unknown-linux-gcc: Command not found
  HOSTCC  scripts/basic/fixdep
make: h8300-unknown-linux-gcc: Command not found
  WRAP    arch/h8300/include/generated/uapi/asm/kvm_para.h
  [ snip ]

The solution is to delete this line, or to use cc-cross-prefix like
some architectures do. I chose the latter as a moderate fixup.

I added an alternative 'h8300-linux-' because it is available at:

https://mirrors.edge.kernel.org/pub/tools/crosstool/files/bin/x86_64/8.1.0/



Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>