Attempting to build mach-rpc with gcc-9 or higher, or with any version
of clang results in a build failure, like:

arm-linux-gnueabi-gcc-11.1.0: error: unrecognized -march target: armv3m
arm-linux-gnueabi-gcc-11.1.0: note: valid arguments are: armv4 armv4t armv5t armv5te armv5tej armv6 armv6j armv6k armv6z armv6kz armv6zk armv6t2 armv6-m armv6s-m armv7 armv7-a armv7ve armv7-r armv7-m armv7e-m armv8-a armv8.1-a armv8.2-a armv8.3-a armv8.4-a armv8.5-a armv8.6-a armv8-m.base armv8-m.main armv8-r armv8.1-m.main iwmmxt iwmmxt2; did you mean 'armv4'?

Building with gcc-5 also fails in at least one of these ways:

/tmp/cczZoCcv.s:68: Error: selected processor does not support `bx lr' in ARM mode
drivers/tty/vt/vt_ioctl.c:958:1: internal compiler error: Segmentation fault

Handle this in Kconfig so we don't run into this with randconfig
builds, allowing only gcc-6 through gcc-8.
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>