native.bbclass: make TOOLCHAIN_OPTIONS empty

native.bbclass: make TOOLCHAIN_OPTIONS empty

The native pkg doesn't need the TOOLCHAIN_OPTIONS, but it maybe used by
native pkgs, for example, the cmake.bbclass uses TOOLCHAIN_OPTIONS:

OECMAKE_C_FLAGS ?= "${HOST_CC_ARCH} ${TOOLCHAIN_OPTIONS} ${CPPFLAGS}"

and TOOLCHAIN_OPTIONS is:
TOOLCHAIN_OPTIONS = " --sysroot=${STAGING_DIR_TARGET}"

If a native pkg inherits cmake.bbclass, then:
TOOLCHAIN_OPTIONS = " --sysroot="

and OECMAKE_C_FLAGS would be:
${HOST_CC_ARCH} --sysroot= ${CPPFLAGS}

This would cause unexpected errors, I think that we can make
TOOLCHAIN_OPTIONS empty in native.bbclass since native pkg doesn't need
it.

[YOCTO #2124]

(From OE-Core rev: e5555c743a9637f86a0a2c4c45a63a80838e81ae)

Signed-off-by: Robert Yang <liezhi.yang@windriver.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>