Move ARM64 Assembler::nop definition from header to source file

When Assembler::nop is in the header, it is considered an inline
function. With GN arg is_component_build=true, the V8_EXPORT_PRIVATE
mark on the class causes it to be exported every time the header is
included. This, in turn, produces a reference to
Register::XRegFromCode.

Register::XRegFromCode is only ever defined as an inlined function, so
that reference is never fulfilled.

Clang can avoid this using the /Fc:dllexportInlines- flag to suppress
the export of Assembler::nop and so avoid generating the reference to
Register::XRegFromCode.

MSVC does not support this flag, so this change suppresses the export
by moving Assembler::nop's definition to the .cc file. This also allows
it to use the inline definition of Register::XRegFromCode.

Bug: v8:8870
Change-Id: I1cd33195677256c9dd06c7047fe84e1b912d3151
Reviewed-on: https://chromium-review.googlesource.com/c/1478216Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Commit-Queue: Michael Achenbach <machenbach@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59785}

src/arm64/assembler-arm64.cc

@@ -1105,6 +1105,12 @@ void Assembler::adr(const Register& rd, Label* label) {
 }
 
 
+void Assembler::nop(NopMarkerTypes n) {
+  DCHECK((FIRST_NOP_MARKER <= n) && (n <= LAST_NOP_MARKER));
+  mov(Register::XRegFromCode(n), Register::XRegFromCode(n));
+}
+
+
 void Assembler::add(const Register& rd,
                     const Register& rn,
                     const Operand& operand) {

src/arm64/assembler-arm64.h

@@ -1095,10 +1095,7 @@ class V8_EXPORT_PRIVATE Assembler : public AssemblerBase {
     LAST_NOP_MARKER = ADR_FAR_NOP
   };
 
-  void nop(NopMarkerTypes n) {
-    DCHECK((FIRST_NOP_MARKER <= n) && (n <= LAST_NOP_MARKER));
-    mov(Register::XRegFromCode(n), Register::XRegFromCode(n));
-  }
+  void nop(NopMarkerTypes n);
 
   // Add.
   void add(const VRegister& vd, const VRegister& vn, const VRegister& vm);