FTL arity fixup should work on ARM64

FTL arity fixup should work on ARM64
https://bugs.webkit.org/show_bug.cgi?id=129810

Reviewed by Michael Saboff.

- Using regT5 to pass the thunk return address to arityFixup is shady since that's a
  callee-save.

- The FTL path was assuming X86 conventions for where SP points at the top of the prologue.

This makes some more tests pass.

* dfg/DFGJITCompiler.cpp:
(JSC::DFG::JITCompiler::compileFunction):
* ftl/FTLLink.cpp:
(JSC::FTL::link):
* jit/AssemblyHelpers.h:
(JSC::AssemblyHelpers::prologueStackPointerDelta):
* jit/JIT.cpp:
(JSC::JIT::privateCompile):
* jit/ThunkGenerators.cpp:
(JSC::arityFixup):
* llint/LowLevelInterpreter64.asm:
* offlineasm/arm64.rb:
* offlineasm/x86.rb: In addition to the t7 change, make t6 agree with GPRInfo.h.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@165205 268f45cc-cd09-0410-ab3c-d52691b4dbfc