scripts: ext-toolchain: fix wrong prefix in print_config generation

scripts: ext-toolchain: fix wrong prefix in print_config generation

The parsed prefix in print_config is wrong and this produce broken
generated .config that won't work with any external toolchain.

Currently the prefix from a CC of

'arm-openwrt-linux-muslgnueabi-gcc-12.1.0'

produce a prefix

'arm-openwrt-linux-muslgnueabi-gcc-'

This is wrong as the real prefix should be

'arm-openwrt-linux-muslgnueabi-'

This is probably caused by a change in how the toolchain is now handled
that now append also the gcc version. Probably in ancient days the
version wasn't part of the name and the prefix generation stripped the
'-gcc' instead of the gcc version.

Fix this and correctly strip the gcc version and the gcc suffix to
correctly call toolchain bins.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>