Also brutally silence -Wcast-align warnings in OS_MBUF_PKTHDR()
and OS_MBUF_PKTHDR_TO_MBUF(), as those are used by nimble users.
It might make sense to check if the silenced / disabled warnings do
indeed point out unaligned memory accesses.
Nimble contains a couple of casts that discard alignment information.
This causes a warning with clang's -Wno-address-of-packed-member.
A previous PR (#10503) supressed that warning. This commit re-enables
them and provides a patch to fix the offending code.
The fix has been submitted upstream, see
https://github.com/apache/mynewt-nimble/pull/252
