math: fix missing attributes warnings

gcc-9 started warning if alias symbols have different attributes:

  math/expf.c: At top level:
  math/expf.c:89:21: warning: '__expf_finite' specifies less restrictive attributes than its target 'expf': 'leaf', 'nothrow', 'pure' [-Wmissing-attributes]

so copy the attributes when creating the aliases.