Use the musl versions of for the remainder libm functions

[arichardson]

Summary

This fixes rem_test on AArch64 and should be mostly a no-op for x86 since
we use assembly versions there. I did not update the remquol()
implementation, since the musl version fails rem_test whereas the current
one passes. Similarly modfl() was also not updated since the musl version
raises a spurious FE_INEXACT that the tests do not like.

Test Plan

rem_test now passes on aarch64

[arichardson]

Drop modfl update