toolchain/gcc: switch to version 7 by default

toolchain/gcc: switch to version 7 by default

Newer GCC versions are reported to improve code size on various ARM
devices, which is why newer u-boot versions require them now.
There have also been user reports of data corruption issues fixed on
MIPS by updating from GCC 5 to GCC 7.

This update was previously held up by these two MIPS compile issues,
which have since been fixed:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=83496
https://gcc.gnu.org/bugzilla/show_bug.cgi?id=84790

Signed-off-by: Felix Fietkau <nbd@nbd.name>