RFR: JDK-8148655 LOG=cmdlines and other build-infra fixes

[Magnus Ihse Bursie]

This is yet another collection of fixes from the build-infra hotspot 
project forest that has a stand-alone value.

The most important change is the support of a new log option, cmdlines. 
This is, like the old "nofile", an option that can be added to a log 
level, e.g. "LOG=info,cmdlines" or used standalone "LOG=cmdlines" (in 
which case the log level stays at default). With this in place, the 
command line of "important" commands are printed. Examples of 
"important" commands are compiler and linker calls. Examples of 
"non-important" commands are "mkdir" or "cat". Note that at this point, 
not all "important" calls are identified, typically in esoteric stuff 
like gensrc.

Apart from this, a few other changes are also included:
* Allow DEBUG_SYMBOLS to be individually turned off (follow up to 
JDK-8145596)
* Support .S assembly files
* Expose USERNAME outside configure
* Fix broken indentation

Bug: https://bugs.openjdk.java.net/browse/JDK-8148655
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8148655-LOG-cmdlines-and-misc-fixes/webrev.01

/Magnus