Fix TARGET_CHAR_BIT/HOST_CHAR_BIT confusion in gmp-utils.c

Fix TARGET_CHAR_BIT/HOST_CHAR_BIT confusion in gmp-utils.c

In a couple of gdb_mpz methods, we are computing the number of
bits in a gdb::array_view of gdb_byte. Since gdb_byte is defined
using a host-side type (see common-types.h), the number of bits
in a gdb_byte should be HOST_CHAR_BIT, not TARGET_CHAR_BIT.

gdb/ChangeLog:

        * gmp-utils.c (gdb_mpz::read): Use HOST_CHAR_BIT instead of
        TARGET_CHAR_BIT.
        (gdb_mpz::write): Likewise.