Disable memtag sanitization for global fnptrs going into .ctors (#70186)

Looks like there's code out there that, instead of using
'__attribute__((constructor(x)))' to add constructor functions, they
just declare a global function pointer and use
'__attribute__((section('.ctors')))' instead.

Problem is, with memtag-globals, we pad the global function pointer to
be 16 bytes large. This of course means we have an 8-byte real function
pointer, then 8 bytes of zero padding, and this trips up the loader when
it processes this section.

Fixes #69939