Fix amd64 dwarf register number mapping (MMX register and higher)

Fix amd64 dwarf register number mapping (MMX register and higher)

Dwarf register numbers are defined in "System V Application Binary
Interface AMD64 Architecture Processor Supplement Draft Version 0.99.6"

The amd64_dwarf_regmap array is missing the 8 MMX registers in Figure
3.36: DWARF Register Number Mapping page 57.  This leads to a wrong
value for the registers past this point.

gdb/ChangeLog:

        Pushed by Joel Brobecker  <brobecker@adacore.com>.
        * amd64-tdep.c (amd64_dwarf_regmap array): Add missing MMX
        registers.

Tested on x86_64-linux.