ICU-20842 Fix library names in pkg-config files on Windows (mingw-w64)

ICU-20842 Fix library names in pkg-config files  on Windows (mingw-w64)

since the move of the DLL to bin/ the library names in .pc files is
wrong. With ICU 65.1, icu-uc.pc contains

Libs: -L${libdir} -licuuc65 -licudt65

the version number should not appear. Indeed, the linker looks for the
libraries in $prefix/lib in the following order (see [1]):

libxxx.dll.a
xxx.dll.a
libxxx.a
cygxxx.dll
libxxx.dll
xxx.dll

As the is only the import library with no versioning (which is normal),
the is a link error when using ICU pc files.

[1] https://access.redhat.com/documentation/en-US/Red_Hat_Enterprise_Linux/4/html/Using_ld_the_GNU_Linker/win32.html