Install libm.a as linker script (bug 20539).

Install libm.a as linker script to avoid static link fail w/o passing
additional -lmvec option while building with GCC >= 6.1.

    [BZ #20539]
    * math/Makefile (install-lib-ldscripts): Add libm.a.
    (install_subdir): Remove.
    (install-others): Add.
    ($(inst_libdir)/libm.a): Add rule for installation as
    linker script.
    * Makerules (install-lib.a): Filter out install-lib-ldscripts.