fix: avoid macOS linker error for rust_psm_on_stack_end

On macOS targets, don't define the symbol `rust_psm_on_stack_end`
since it isn't used (and somehow this causes the build to fail).

Fixes #87

psm/src/arch/aarch_aapcs64.s

@@ -7,20 +7,23 @@
 #define GLOBL(fnname) .globl _##fnname
 #define TYPE(fnname)
 #define FUNCTION(fnname) _##fnname
+#define LABEL_FOR_SIZE(endlabel)
 #define SIZE(fnname,endlabel)
 
 #elif CFG_TARGET_OS_windows
 
 #define GLOBL(fnname) .globl fnname
 #define TYPE(fnname)
 #define FUNCTION(fnname) fnname
+#define LABEL_FOR_SIZE(endlabel)
 #define SIZE(fnname,endlabel)
 
 #else
 
 #define GLOBL(fnname) .globl fnname
 #define TYPE(fnname) .type fnname,@function
 #define FUNCTION(fnname) fnname
+#define LABEL_FOR_SIZE(endlabel) endlabel:
 #define SIZE(fnname,endlabel) .size fnname,endlabel-fnname
 
 #endif
@@ -87,6 +90,6 @@ FUNCTION(rust_psm_on_stack):
     .cfi_restore x29
     .cfi_restore x30
     ret
-.rust_psm_on_stack_end:
+LABEL_FOR_SIZE(rust_psm_on_stack_end)
 SIZE(rust_psm_on_stack,.rust_psm_on_stack_end)
 .cfi_endproc

psm/src/arch/x86_64.s

@@ -8,13 +8,15 @@
 #define GLOBL(fnname) .globl _##fnname
 #define TYPE(fnname)
 #define FUNCTION(fnname) _##fnname
+#define LABEL_FOR_SIZE(endlabel)
 #define SIZE(fnname,endlabel)
 
 #else
 
 #define GLOBL(fnname) .globl fnname
 #define TYPE(fnname) .type fnname,@function
 #define FUNCTION(fnname) fnname
+#define LABEL_FOR_SIZE(endlabel) endlabel:
 #define SIZE(fnname,endlabel) .size fnname,endlabel-fnname
 
 #endif
@@ -82,6 +84,6 @@ FUNCTION(rust_psm_on_stack):
     popq  %rbp
     .cfi_def_cfa %rsp, 8
     retq
-.rust_psm_on_stack_end:
+LABEL_FOR_SIZE(rust_psm_on_stack_end)
 SIZE(rust_psm_on_stack,.rust_psm_on_stack_end)
 .cfi_endproc