RFR: 8202978: Incorrect tmp register passed to MacroAssembler::load_mirror()

[Per Liden]

On x86, MacroAssembler::load_mirror() defaults to using rscratch2 as tmp 
register, unless something else is specified. In 
TemplateInterpreterGenerator::generate_native_entry() we call 
load_mirror(), but the rscratch2 register is already in use in this 
context, which obviously leads to problems. This is not a performance 
critical path, so we should just pass noreg as the tmp register.

Bug: https://bugs.openjdk.java.net/browse/JDK-8202978
Webrev: http://cr.openjdk.java.net/~pliden/8202978/webrev.0

Testing: passes hs-tier{1,2}

/Per