Various changes to the registers displayed in DDB for x86.

Various changes to the registers displayed in DDB for x86.
- Fix segment registers to only display the low 16 bits.
- Remove unused handlers and entries for the debug registers.
- Display xcr0 (if valid) in 'show sysregs'.
- Add '0x' prefix to MSR values to match other values in 'show sysregs'.
- MFamd64: Display various MSRs in 'show sysregs'.
- Add a 'show dbregs' to display the value of debug registers.
- Dynamically size the column width for register values to properly
  align columns on 64-bit platforms.
- Display %gs for i386 in 'show registers'.

Differential Revision:	https://reviews.freebsd.org/D2784
Reviewed by:	kib, markj
MFC after:	2 weeks