crt: Add _copysignf into all i386 CRT libraries

Function _copysignf is in i386 natively available only in msvcr120 and
UCRT. mingw-w64 already provides emulation in crtdll and msvcrt i386 import
libraries. Include this emulation also into all other i386 CRT import
libraries.

Note that function _copysignf is available in all x64 and arm versions of
msvcrt libraries. So ensure that i386 emulation is not included into
non-i386 builds by adding #if guard for the whole source file. So when it
is compiled for non-i386 build, it would not provide any symbol or
function. Also rename this file to i386__copysignf.c, so it will be clear
that this file i386-only.

Signed-off-by: Martin Storsjö <martin@martin.st>