RFR(S): 8148159: [TESTBUG] TestCompilerDirectivesCompatibility tests fails on non-tiered server VMs

[Nils Eliasson]

Hi,

Please review this change.

The tests relied on C1 always being available and thus fails on C2 only 
server configurations. The fix adds two whitebox methods for checking if 
C1 and C2 is available, and then tests both compilers separately. Also 
adding Xmixed to the commandline since this test doesn't need Xcomp testing.

Bug:
https://bugs.openjdk.java.net/browse/JDK-8148159

Webrev:
http://cr.openjdk.java.net/~neliasso/8148159/webrev.01/
http://cr.openjdk.java.net/~neliasso/8148159/webrev_jdk.01/

Regards,
Nils Eliasson