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.