efi_loader: variables PlatformLang and PlatformLangCodes

Since TianoCore EDK2 commit d65f2cea36d1 ("ShellPkg/CommandLib: Locate
proper UnicodeCollation instance") in edk2 the UEFI Shell crashes if EFI
variable PlatformLang is not defined.

As this variable is anyway prescribed in the UEFI 2.7 spec let's define it
to L"en-US". Use the same value for PlatformLangCodes that defines the list
of all supported languages.

Signed-off-by: Heinrich Schuchardt <xypron.glpk@gmx.de>