gdb: fix -Wsingle-bit-bitfield-constant-conversion warning in z80-tdep.c

gdb: fix -Wsingle-bit-bitfield-constant-conversion warning in z80-tdep.c

When building with clang 16, I see:

    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:338:32: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
            info->prologue_type.load_args = 1;
                                          ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:345:36: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.critical = 1;
                                       ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:351:37: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.interrupt = 1;
                                        ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:367:36: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
                  info->prologue_type.fp_sdcc = 1;
                                              ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:375:35: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.fp_sdcc = 1;
                                      ^ ~
    /home/smarchi/src/binutils-gdb/gdb/z80-tdep.c:380:35: error: implicit truncation from 'int' to a one-bit wide bit-field changes value from 1 to -1 [-Werror,-Wsingle-bit-bitfield-constant-conversion]
          info->prologue_type.fp_sdcc = 1;
                                      ^ ~

Fix that by using "unsigned int" as the bitfield's underlying type.

Change-Id: I3550a0112f993865dc70b18f02ab11bb5012693d