xtensa: move CONFIG_CPU_*_ENDIAN defines to Kconfig

Move the definition of CONFIG_CPU_*_ENDIAN to Kconfig, the best place
for CONFIG options.

I slightly simplified the test code. You can use the -P option to suppress
linemarker generation. The grep command is unneeded.

  $ echo __XTENSA_EB__ | xtensa-linux-gcc -E -
  # 1 "<stdin>"
  # 1 "<built-in>"
  # 1 "<command-line>"
  # 1 "<stdin>"
  1

  $ echo __XTENSA_EB__ | xtensa-linux-gcc -E -P -
  1

Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>
Message-Id: <20210313122342.69995-1-masahiroy@kernel.org>
Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>