acosf: Fix when building with gcc <= 11

GCC <= 11 wrongly assumes the rounding is to nearest and performs a constant folding where it should evaluate since the result is not exact [1].

This is similar to the fix I proposed on glibc [2].

Tested on aarch64 with ubuntu 24.04:

$ CC=gcc-11 CFLAGS=-O3 ./check.sh acosf Running exhaustive check in --rndn mode... all ok Running exhaustive check in --rndz mode... all ok Running exhaustive check in --rndu mode... all ok Running exhaustive check in --rndd mode... all ok

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=57245 [2] https://patchwork.sourceware.org/project/glibc/patch/20250102143444.3574078-1-adhemerval.zanella@linaro.org/