x84_64: It's only 'mcontext_t'.
Fixes warning:
```
src/x86_64/Gos-linux.c:141:36: warning: initialization from incompatible pointer type
struct mcontext_t *sc_mcontext = &((struct ucontext*)sc)->uc_mcontext;
^
```
Introduced-in:
@@ -138,7 +138,7 @@ x86_64_sigreturn (unw_cursor_t *cursor)
{
struct cursor *c = (struct cursor *) cursor;
struct sigcontext *sc = (struct sigcontext *) c->sigcontext_addr;
- struct mcontext_t *sc_mcontext = &((struct ucontext*)sc)->uc_mcontext;
+ mcontext_t *sc_mcontext = &((struct ucontext*)sc)->uc_mcontext;
/* Copy in saved uc - all preserved regs are at the start of sigcontext */
memcpy(sc_mcontext, &c->uc->uc_mcontext,
DWARF_NUM_PRESERVED_REGS * sizeof(unw_word_t));
