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: NMartin Storsjö <martin@martin.st>

configure

@@ -2823,7 +2823,7 @@ probe_cc(){
     elif $_cc --vsn 2>/dev/null | grep -Eq "ARM (C/C\+\+ )?Compiler"; then
         test -d "$sysroot" || die "No valid sysroot specified."
         _type=armcc
-        _ident=$($_cc --vsn | head -n1 | sed 's/.*: //')
+        _ident=$($_cc --vsn | grep -i build | head -n1 | sed 's/.*: //')
         armcc_conf="$PWD/armcc.conf"
         $_cc --arm_linux_configure                 \
              --arm_linux_config_file="$armcc_conf" \