Probe for the toolchain default architecture version if no --cpu flag
is present or an unknown cpu is specified.  Works with gcc, clang and
armcc.

This allows configuring based on the arch version even if it is not
explicitly specified to configure.  It also causes an explicit -march
flag to be added to CFLAGS and ASFLAGS, which in turn lets us do
proper instruction set tests with the assembler.
Signed-off-by: Mans Rullgard <mans@mansr.com>