pkg: Allocate a suitably-sized string for the local ABI

Previously the local ABI string was written to an on-stack buffer and
the pointer to that buffer was saved in a global before the function
returned. This had two issues: c[ABI].val pointed to a
no-longer-valid on-stack buffer after config_init returned, and the
string could potentially be truncated. Fix both of those by changing
pkg_get_myabi to return a pointer to a string allocated by asprintf.

Note that the allocated string is left in the global config array
until it is implicitly freed on process exit.

Reported by: GCC 13 -Wdangling-pointer
Reviewed by: emaste
Differential Revision: https://reviews.freebsd.org/D42623