Rename the `ALIGN' macro to `UNW_ALIGN', and move it from
`_UCD_internal.h' to `libunwind_i.h' so that we can share it with the
mempool code. `ALIGN' was clashing with system headers on FreeBSD:
In file included from src/coredump/_UCD_access_reg_freebsd.c:26:
src/coredump/_UCD_internal.h:102:1: warning: "ALIGN" redefined
In file included from /usr/include/sys/param.h:115,
from src/coredump/_UCD_lib.h:52,
from src/coredump/_UCD_access_reg_freebsd.c:24:
/usr/include/machine/param.h:79:1: warning: this is the location of the previous definition
ALIGN lets you align pointers and STRUCT_MEMBER lets you get
structure members at a specific offset.
These are useful in general, and will be needed for the coredump notes
cleanup work.