[flang] Enable target rewrite for int args/rets of bind(c) functions.

BIND(C) subprograms must use the same target ABI as the C processor,
so 1/2-byte int args/rets must be rewritten to use signext attribute.
This change-set also sets fir.bindc_name for the math functions used
during lowering so that they can be fixed up as well (though, currently
none of them needs to be affected).

Differential Revision: https://reviews.llvm.org/D145537