crosssdk: Remove usage of host flags for cross-compilation

crosssdk: Remove usage of host flags for cross-compilation

Similarlly to OE-Core rev 4b936cde58ca0a6f34092ce82640a02859110411 for
cross.sdk, BUILD_* flags can't be used as TARGET_* flags

gcc-crosssdk buils leaks config.log's through "gcc-stashed-builddir" and
TARGET_* flags to libgcc cross-build through "gcc/libgcc.mvars" file
on "gcc-stashed-builddir". This means that if BUILD_CFLAGS contains
host-specific flags like "-isystem/usr/include" libgcc build will
fail "do_qa_configure" and "do_package_qa" checks.

Remove host-related flags from TARGET_* flags for gcc-crosssdk builds.

[YOCTO #11874]

(From OE-Core rev: 6e162e619b6f5173c073cd9bedbcadf205017e30)

Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>