Work around x86-64 calling convention bug

Work around x86-64 calling convention bug

Microsoft's x86-64 calling convention ABI requires registers XMM6-15 to
be preserved by the callee:
https://docs.microsoft.com/en-us/cpp/build/x64-calling-convention?view=vs-2017#calling-convention-defaults
Currently only the System V calling convention's behavior is supported,
which does not have any callee-saved XMM registers.

Fixing this properly is non-trivial and this bug is blocking other
projects, so a temporary workaround is to limit Subzero to use only
scratch registers XMM0-XMM5.

Bug chromium:931926
Bug swiftshader:22

Change-Id: If3fde2295fc65fc52042124afe7dfc46873cce3f
Reviewed-on: https://swiftshader-review.googlesource.com/c/25568
Tested-by: Nicolas Capens <nicolascapens@google.com>
Reviewed-by: Alexis Hétu <sugoi@google.com>