RFR: 8081676 Verify that configure detects AS on Solaris and print help otherwise

[Tim Bell]

Hello

Please review this added sanity check for the Solaris assembler (as).

'as' is not installed by default on Solaris, and it would be better to 
fail fast if missing.

Bug report:
   https://bugs.openjdk.java.net/browse/JDK-8081676

Webrev:
   http://cr.openjdk.java.net/~tbell/8081676/

Note: This is a change to toolchain.m4.  I did not include the 
generated-configure.sh files in the webrev, but I will regenerate and 
push them as well.

Thank you in advance-

Tim