BaseTools UNIXGCC ELFGCC CYGGCC: clone GCC build rule family into GCCLD

BaseTools UNIXGCC ELFGCC CYGGCC: clone GCC build rule family into GCCLD

Before we can make non-backward compatible changes to the GCC build rules
regarding the use of the 'gcc' binary as the linker, clone the existing
GCC build rules into a 'GCCLD' build rule family, and move the legacy
toolchains UNIXGCC, CYGGCC, CYGGCCxASL and ELFGCC over to it.

Contributed-under: TianoCore Contribution Agreement 1.0
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Reviewed-by: Jordan Justen <jordan.l.justen@intel.com>
Reviewed-by: Liming Gao <liming.gao@intel.com>