newlib: remove __infinity{f,ld} constants

newlib: remove __infinity{f,ld} constants

previous commit 4c90db7bc89e7fa1077025fefdd58269dc71a6ac introduced
a compile time error because libm/common/s_infconst.c used the remove
__fmath, __dmath, and __ldmath union types.

Since this is very old, and unused for a very long time, just drop the
file and thus the __infinity constants entirely.

Exception: Cygwin exports __infinity from the beginning.  There's a very,
VERY low probability that any existing executable or lib still uses this
constant, but we just keep it in for backward compat, nevertheless.

Signed-off-by: Corinna Vinschen <corinna@vinschen.de>