Rather than detecting vfp support via catching SIGILL signals,
parse the contents of /proc/cpuinfo.
As the GOARM values for NaCl and freebsd are hard coded, this parsing
logic only needs to support linux/arm.
This change also fixes the nacl/arm build which is broken because the
first stage of nacltest.bash is executed with GOARM=5, embedding that
into 5g.
The second stage of nacltest.bash correctly detects GOARM=7, but this is
ignored as we pass --no-clean at that point, and thus do not replace
the compiler.
Lastyly, include a fix to error message in nacltest.bash