Also make the functions constexpr.

Due to slight changes in floating-point arithmetics (90.0 instead of
180.0, M_PI2 instead of M_PI resp.), results might differ in last
digits (usually 17th decimal digit). This has lead to need to tweak
char2dump's PieChartTest unit test.

Change-Id: I20323dd7dab27e4deb408ea4181e390cc05e7cd3
Reviewed-on: https://gerrit.libreoffice.org/58583
Tested-by: Jenkins
Reviewed-by: Tamás Zolnai <tamas.zolnai@collabora.com>
Reviewed-by: Mike Kaganski <mike.kaganski@collabora.com>