unknown token in expression in GC code for ARM64 arch

Setting flag to `HXCPP_ARCH=arm64` on Mac M1 causes the following problem:

```
Error: /usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:69:12: error: unknown token in expression
asm ("movq %%rbx, %0\n\t" : "=r" (regBx) );
^
:1:7: note: instantiated into assembly here
movq %rbx, x8
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:69:12: error: invalid operand
asm ("movq %%rbx, %0\n\t" : "=r" (regBx) );
^
:1:7: note: instantiated into assembly here
movq %rbx, x8
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:70:12: error: unknown token in expression
asm ("movq %%rbp, %0\n\t" : "=r" (regBp) );
^
:1:7: note: instantiated into assembly here
movq %rbp, x9
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:70:12: error: invalid operand
asm ("movq %%rbp, %0\n\t" : "=r" (regBp) );
^
:1:7: note: instantiated into assembly here
movq %rbp, x9
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:72:12: error: unknown token in expression
asm ("movq %%r12, %0\n\t" : "=r" (reg12) );
^
:1:7: note: instantiated into assembly here
movq %r12, x8
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:72:12: error: invalid operand
asm ("movq %%r12, %0\n\t" : "=r" (reg12) );
^
:1:7: note: instantiated into assembly here
movq %r12, x8
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:73:12: error: unknown token in expression
asm ("movq %%r13, %0\n\t" : "=r" (reg13) );
^
:1:7: note: instantiated into assembly here
movq %r13, x9
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:73:12: error: invalid operand
asm ("movq %%r13, %0\n\t" : "=r" (reg13) );
^
:1:7: note: instantiated into assembly here
movq %r13, x9
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:74:12: error: unknown token in expression
asm ("movq %%r14, %0\n\t" : "=r" (reg14) );
^
:1:7: note: instantiated into assembly here
movq %r14, x10
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:74:12: error: invalid operand
asm ("movq %%r14, %0\n\t" : "=r" (reg14) );
^
:1:7: note: instantiated into assembly here
movq %r14, x10
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:75:12: error: unknown token in expression
asm ("movq %%r15, %0\n\t" : "=r" (reg15) );
^
:1:7: note: instantiated into assembly here
movq %r15, x11
^
/usr/local/lib/haxe/lib/hxcpp/git/src/hx/gc/GcRegCapture.cpp:75:12: error: invalid operand
asm ("movq %%r15, %0\n\t" : "=r" (reg15) );
^
:1:7: note: instantiated into assembly here
movq %r15, x11
^
12 errors generated.
Error: Build failed
```