coredump: use unaligned_read_ne{32,64}() to parse auxv

coredump: use unaligned_read_ne{32,64}() to parse auxv

Fixes a bug introduced by 3e4d0f6cf99f8677edd6a237382a65bfe758de03.

The auxv metadata is unaligned, as the length of the prefix
"COREDUMP_PROC_AUXV=" is 19. Hence, parse_auxv{32,64}() may triger
an undefined behavior (or at least cause slow down), which can be
detected when running on an undefined behavior sanitizer.

This also introduces a macro to define `parse_auxv{32,64}()`.

Fixes #26912.