Darwin : Make trampoline templates linker-visible.

For Arm64, the alignment of the LTRAMPn symbols matters.

Actually, the LTRAMPn  symbols _are_ 8 byte aligned, but because
they are Local, the linker doesn't know that this guarantee can be met.
It assumes that they are not necessarily more aligned than the
containing section (ld64 atoms strike again).

The fix is to publish the trampoline symbol for the linker to access
directly - it can then see that the atom is suitably aligned.

Fixes issue #11.

gcc/config/darwin.h

@@ -952,6 +952,8 @@ extern GTY(()) section * darwin_sections[NUM_DARWIN_SECTIONS];
       sprintf (LABEL, "*%s%ld", "lubsan_type", (long)(NUM));\
     else if (strcmp ("LASAN", PREFIX) == 0)	\
       sprintf (LABEL, "*%s%ld", "lASAN", (long)(NUM));\
+    else if (strcmp ("LTRAMP", PREFIX) == 0)	\
+      sprintf (LABEL, "*%s%ld", "lTRAMP", (long)(NUM));\
     else						\
       sprintf (LABEL, "*%s%ld", PREFIX, (long)(NUM));	\
   } while (0)