math/slatec: fix build on armv7

GNU ld supports neither -z relro nor -z norelro on armv7.
Unfortunately <bsd.lib.mk> insists on adding either to the link.
Work around this issue by using base ld for the link.

While we're at it, set LICENSE=PD (US government work).

MFH: 2025Q2
(cherry picked from commit f56f31e07cb64c0b183127733bc6cd6af61efea1)