Fix version detection for recent gcc

GCC 7 in Ubuntu 18 and GCC 9 in Ubuntu 20
only emits the major version when called
with -dumpversion which causes GCC_MINOR_VERSION
to become empty. GCC also has a -dumpfullversion
that will emit the entire version on GCC 7
and later, but msp430-gcc will fail with
an error message because there are no input
files.

Adding both parameters makes both msp430-gcc
and GCC 7/9 on Ubuntu provide the full version
so use that in the GCC version detection.