Shadow registers wronly shown by gdb on avx machine

Shadow registers wronly shown by gdb on avx machine
For an unclear reason, the orig_rax register and its shadows are described in the
xml file using a register number.
This register number is correct on non avx machine, but is wrong on
avx machine, as these have more registers, which means that orig_raxs1 and s2
should have different numbers.

As no reason was found to have a register number explicitely give, remove this
regnnr from the xml file, and let gdb calculate it.

git-svn-id: svn://svn.valgrind.org/valgrind/trunk@14128