Fix loongarch_iterate_over_regset_sections for non-native targets.

Fix loongarch_iterate_over_regset_sections for non-native targets.

Define a constant for the number of registers stored in a register set
and use this with register_size to compute the size of the
general-purpose register set in core dumps.

This also fixes the build on hosts such as FreeBSD that do not define
an elf_gregset_t type.