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

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
Reviewed By: mmel
Approved by: re (gjb)
Differential Revision: https://reviews.freebsd.org/D28938

(cherry picked from commit 0e4ff0acbe80c547988bede738af2e227c7eb47c)
(cherry picked from commit d37fb0e37814db8f76462b3b9f1fb0e6dfca6324)