awk: temporarily fix for NaN() exceptions on 386

problem is NaN() produces a SNaN, not a QNaN... and on the 387,
storing 80 bit SNaN in register to a 64-bit memory destination
traps.

SNaN/QNaN encoding is machine specific. mips has the qiet/signaling
bit inverted.

disabling fp exception in main() now, but that sucks.

i think the best solution would be to not even call strtod() in
is_number() but just write a regex or a little state machine that
will only accept numbers without nan and ±inf.

that might even make it faster and is more robust than relying on
the os's strtod() details.

sys/src/cmd/awk/main.c

@@ -55,6 +55,8 @@ void main(int argc, char *argv[])
 	char *fs = nil, *marg;
 	int temp;
 
+	setfcr(getfcr() & ~FPINVAL);
+
 	Binit(&stdin, 0, OREAD);
 	Binit(&stdout, 1, OWRITE);
 	Binit(&stderr, 2, OWRITE);