regcache: Zero-extend small registers described by a register map.

When registers are supplied via regcache_supply_register from a
register block described by a register map, registers may be stored in
slots smaller than GDB's native register size (e.g. x86 segment
registers are 16 bits, but the GDB registers for those are 32-bits).
regcache_collect_regset is careful to zero-extend slots larger than a
register size, but regcache_supply_regset just used
regcache::raw_supply_part and did not initialize the upper bytes of a
register value.

trad_frame_set_reg_regmap assumes these semantics (zero-extending
short registers).  Upcoming patches also require these semantics for
handling x86 segment register values stored in 16-bit slots on
FreeBSD.  Note that architecturally x86 segment registers are 16 bits,
but the x86 gdb architectures treat these registers as 32 bits.