RFR: JDK-8245033 Fixes for building in WSL

[Magnus Ihse Bursie]

Here's a bunch of minor fixes to improve developer experience when 
building on WSL.

The fixes include:
  * Now the FixPath macro works for WSL.
  * TestImage.gmk tried to use FIXPATH for /bin/echo, does not work on WSL
  * Use -t drvfs option to df to properly detect a suitable local disk 
to build on on WSL
  * UTIL_PATH_PROGS for cmd.exe had syntax errors and did not work properly
  * Allow for tools to have a required .exe suffix (as for Windows-based 
tools on WSL)
  * The WSLENV=PATH/l conversion makes WSL produce noisy output at 
times. Suppress this by clearing out the PATH when not needed before 
calling Windows-based tools.
  * Make the VC_VARS extraction less noisy by redirecting output to a 
log file
  * The "AR" tool (i.e. lib.exe on Windows) was not passed the ARFLAGS, 
which included the /nologo. As a result, all calls to lib.exe produced a 
large version banner.
  * Fix JIB configuration to match the fact that "cross-compilation" on 
WSL do not use the cross-compilation names for packages.

I have tested this on WSL1 and Cygwin (to verify that there is no 
regressions). WSL2 is scheduled for public release later in May, and I 
intend to make sure the build works as well with both WSL1 and WSL2 when 
that happens. (For now, I have not opted to test pre-release versions on 
my Windows machine.)

Bug: https://bugs.openjdk.java.net/browse/JDK-8245033
WebRev: 
http://cr.openjdk.java.net/~ihse/JDK-8245033-fix-building-on-WSL/webrev.01

/Magnus