coro: fix dereferencing of `void *` pointer warning

Since the fiber function is not expected to return (i.e., the inline
assembly is not expected to return), remove the dummy memory operand used
to indicate that the memory pointed to by the inline assembly pointer
operand is clobbered.

Closes #8125

NO_DOC=build diagnostic
NO_CHANGELOG=build diagnostic
NO_TEST=build diagnostic

(cherry picked from commit 97a3af1b)