gcov-tool: Flexible endian adjustment for merging coverage data

gcc/ChangeLog:
2020-05-27  Dong JianQiang  <dongjianqiang2@huawei.com>

	PR gcov-profile/95332
	* gcov-io.c (gcov_var::endian): Move field.
	(from_file): Add IN_GCOV_TOOL check.
	* gcov-io.h (gcov_magic): Ditto.

libgcc/ChangeLog:
2020-05-27  Dong JianQiang  <dongjianqiang2@huawei.com>

	PR gcov-profile/95332
	* libgcov-util.c (read_gcda_file): Call gcov_magic.
	* libgcov.h (gcov_magic): Disable GCC poison.