configure: Include the armcc build number in the compiler identification

This tries to find the most expressive part of the output of
armcc --vsn to include, giving a compiler identification of
"ARM Compiler 5.04 update 2 (build 82)" instead of just
"ARM Compiler 5.04" for armcc 5.0.

4.x versions of armcc output the following, for "armcc --vsn":
ARM C/C++ Compiler, RVCT4.0 [Build 925]
For evaluation purposes only
Software supplied by: ARM Limited

ARM C/C++ Compiler, 4.1 [Build 894]
For evaluation purposes only
Software supplied by: ARM Limited

5.0 versions output this:
Product: ARM Compiler 5.04
Component: ARM Compiler 5.04 update 2 (build 82)
Tool: armcc [5040081]
For evaluation purposes only
Software supplied by: ARM Limited

Signed-off-by: Martin Storsjö <martin@martin.st>