Remove '/' directory separator from CYGWIN & UNIXGCC tool specications.

Remove '/' directory separator from CYGWIN & UNIXGCC tool specications.

This allows for the UNIXGCC_*_PETOOLS_PREFIX and CYGWIN_* macros
to potentially have a prefix before the executable name.  This allows
more flexibility for gcc/binutils when the executables include
a prefix.  Some commented examples are shown where this might be used.
For example:
  DEFINE UNIXGCC_IA32_PETOOLS_PREFIX = i386-pc-mingw32-
    or
  DEFINE UNIXGCC_IA32_PETOOLS_PREFIX = ENV(HOME)/programs/gcc/ia32/bin/i686-pc-mingw32-

git-svn-id: https://edk2.svn.sourceforge.net/svnroot/edk2/trunk/edk2@9076 6f19259b-4bc3-4df7-8a09-765794883524