lib/msun/tests: Re-enable csqrt_test on AArch64

The LLVM bug was fixed a long time ago and with D29076 this test actually
passes now.

Reviewed By: andrew
Differential Revision: https://reviews.freebsd.org/D29092

(cherry picked from commit dbb2d6f5e1231a9f6b49efdc720ce97b520da2ba)