Implement __sd_gc_push_registers for ARM64.

Author: deadalnix

platform/arm64/arm64.mak

@@ -0,0 +1,9 @@
+PLATFORM_ARM64_ASM_SRC = $(wildcard platform/arm64/*.asm)
+PLATFORM_ASM_SRC += $(PLATFORM_ARM64_ASM_SRC)
+
+PLATFORM_ARM64_ASM_OBJ = $(PLATFORM_ARM64_ASM_SRC:platform/%.asm=obj/%.o)
+PLATFORM_ASM_OBJ += $(PLATFORM_ARM64_ASM_OBJ)
+
+# Add the objects to expected variables.
+LIBSDRT_PLATFORM_OBJ += $(PLATFORM_ARM64_ASM_OBJ)
+LIBSDMD_PLATFORM_OBJ += $(PLATFORM_ARM64_ASM_OBJ)

platform/arm64/save_regs.asm

@@ -0,0 +1,23 @@
+.global ___sd_gc_push_registers
+
+.section __TEXT,__text
+
+___sd_gc_push_registers:
+; Standard call ABI. x29 is the frame pointer and x30 the link register.
+	sub     sp, sp, #96
+	stp     x29, x30, [sp, #80]
+	add     x29, sp, #80
+; Registers x19-28 are callee saved.
+	stp     x19, x20, [sp, #0]
+	stp     x21, x22, [sp, #16]
+	stp     x23, x24, [sp, #32]
+	stp     x25, x26, [sp, #48]
+	stp     x27, x28, [sp, #64]
+; This method is passed a delegate. x0 contains the context as a first argument
+; and x1, the second argument is the function pointer. x0 do not need any special
+; threatement as it is also the first argument when calling the delegate.
+	blr x1
+; Restore the frame pointer and link register and return.
+	ldp     x29, x30, [sp, #80]
+	add     sp, sp, #96
+	ret

platform/platform.mak

@@ -18,6 +18,8 @@ endif
 
 ifeq ($(ARCH),x86_64)
 	include platform/x64/x64.mak
+else ifeq ($(ARCH),arm64)
+	include platform/arm64/arm64.mak
 endif
 
 # FIXME: depfiles.

sdlib/d/gc/stack.d

@@ -2,13 +2,7 @@ module d.gc.stack;
 
 import d.gc.types;
 
-version(OSX) {
-	// For some reason OSX's symbol get a _ prepended.
-	extern(C) void _sd_gc_push_registers(void delegate());
-	alias __sd_gc_push_registers = _sd_gc_push_registers;
-} else {
-	extern(C) void __sd_gc_push_registers(void delegate());
-}
+extern(C) void __sd_gc_push_registers(void delegate());
 
 @property
 void* SkipScanStack() {