rt: Remove leading underscore on Win64

Win64 convention does not use underscore.

Author: klutzy

src/rt/arch/x86_64/_context.S

@@ -54,7 +54,7 @@ First four arguments:
         anyhow.
 */
 
-#if defined(__APPLE__) || defined(_WIN32)
+#if defined(__APPLE__)
 #define SWAP_REGISTERS _swap_registers
 #else
 #define SWAP_REGISTERS swap_registers

src/rt/arch/x86_64/ccall.S

@@ -18,10 +18,13 @@
 
         .text
 
-#if defined(__APPLE__) || defined(_WIN32)
+#if defined(__APPLE__)
 .globl ___morestack
 .private_extern MORESTACK
 ___morestack:
+#elif defined(_WIN32)
+.globl __morestack
+__morestack:
 #else
 .globl __morestack
 .hidden __morestack

src/rt/arch/x86_64/morestack.S

@@ -11,7 +11,7 @@
 
 .text
 
-#if defined(__APPLE__) || defined(_WIN32)
+#if defined(__APPLE__)
 #define UPCALL_NEW_STACK        _upcall_new_stack
 #define UPCALL_DEL_STACK        _upcall_del_stack
 #define MORESTACK               ___morestack