bdwgc: bump to version 7.4.2

The code in gcconfig.h indicates that the list of supported architectures is
now a superset of BR2_PACKAGE_LIBATOMIC_ARCH_SUPPORTS so we can drop
BR2_PACKAGE_BDWGC_ARCH_SUPPORTS.

Also, add a hash file.

Cc: Pedro Aguilar <paguilar@paguilar.org>
Signed-off-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>