AArch64: Don't set flush-subnormals-to-zero flag on startup

[arichardson]

Summary

This flag has been set on startup since 65618fdda0f272a823e6701966421bdca0efa301.
However, This causes some of the math-related tests to fail as they report
zero instead of a tiny number. This fixes at least
/usr/tests/lib/msun/ldexp_test and possibly others.
Additionally, setting this flag prevents printf() from printing subnormal
numbers in decimal form.
See also https://www.openwall.com/lists/musl/2021/02/26/1

PR: 253847

[arichardson]

typo fix

[arichardson]

sys/arm64/arm64/vm_machdep.c

58

I just re-ran the lib/msun tests with this set to zero and this results in one fewer failure. Why is this non-default flag being set?

[mmel]

My original idea was to do as much as possible for armv7. Primarily because it has been working on this platform for a long time. My bad is that I didn't remember that flush-to-zero mode was chosen because the little version of armv7 VFP may need software emulation for rounding to denormal values, so we chose the IEEE 754 incompatible mode.

sys/arm64/arm64/vm_machdep.c

58

Bad is that I'm unable to determine which value of DN should be compatible with IEEE 754 standard. By my reading, IEEE754 allows both variants.