[build] Use -fshort-wchar when building EFI host utilities

[build] Use -fshort-wchar when building EFI host utilities

The EFI host utilities (such as elf2efi64, efirom, etc) include the
EDK2 headers, which include static assertions to ensure that they are
built with -fshort-wchar enabled.  When building the host utilities,
we currently bypass these assertions by defining MDE_CPU_EBC.  The EBC
compiler apparently does not support static assertions, and defining
MDE_CPU_EBC therefore causes EDK2's Base.h to define STATIC_ASSERT()
as a no-op.

Newer versions of the EDK2 headers omit the check for MDE_CPU_EBC (and
will presumably therefore fail to build with the EBC compiler).  This
causes our host utility builds to fail since the static assertion now
detects that we are building with the host's default ABI (i.e. without
enabling -fshort-wchar).

Fix by enabling -fshort-wchar when building EFI host utilities.  This
produces binaries that are technically incompatible with the host ABI.
However, since our host utilities never handle any wide-character
strings, this nominal ABI incompatiblity has no effect.

Signed-off-by: Michael Brown <mcb30@ipxe.org>