crossgdb-1.0.tcl unecessary limits arm-none-eabi-gdb to x86-64 arch

[blu (Martin Krastev)]

crossgdb-1.0.tcl explicitly limits building of cross-gdb to x86-64 hosts, due to the fact gdb does not currently support target of macos/arm64.

But that is really not the case with cross-gdb and e.g. arm-none-eabi target -- this is non-native gdb, and as such builds fine on macos. One can successfully build and install an arm-none-eabi-gdb port just by removing the supported_archs limitation in crossgdb-1.0.tcl on Big Sur arm64.