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