Only require libmanagement_ext if it's actually needed.

[zakkak]

On some systems/apps, libawt_headless gets pulled in. This currently brings in libmanagement_ext unconditionally. However, libmanagement_ext should only be present for linking iff
com.sun.management.internal.OperatingSystemImpl class becomes reachable.

Backports: oracle#5122
Closes: #437

[jerboaa]

LGTM

[jerboaa]

The following mandrel IT test failure should be fixed with #445:

 Error:    DebugSymbolsTest.debugSymbolsSmokeGDB:137->carryOutGDBSession:388 There were errors in the GDB session. Note that commands in the session might depend on each other. Errors: 
Command 'info functions .*smoke.*' did not match the expected pattern '.*File debug_symbols_smoke/ClassA.java:.*java.lang.String \*debug_symbols_smoke.ClassA::toString\(\).*File debug_symbols_smoke/Main.java:.*void debug_symbols_smoke.Main\$\$Lambda\$.*::accept\(java.lang.Object\*\).*void debug_symbols_smoke.Main::lambda\$thisIsTheEnd\$0\(java.io.ByteArrayOutputStream\*, debug_symbols_smoke.ClassA\*\).*void debug_symbols_smoke.Main::main\(java.lang.String\[\]\*\).*void debug_symbols_smoke.Main::thisIsTheEnd\(java.util.List\*\).*'.
Output was:
(gdb) All functions matching regular expression ".*smoke.*":

File debug_symbols_smoke/ClassA.java:
37:	java.lang.String *debug_symbols_smoke.ClassA::toString(debug_symbols_smoke.ClassA *);

File debug_symbols_smoke/Main.java:
	
    void debug_symbols_smoke.Main$$Lambda$720/0x00000007c07e8418::accept(debug_symbols_smoke.Main$$Lambda$720/0x00000007c07e8418 *, java.lang.Object *);
50:	
    void debug_symbols_smoke.Main::lambda$thisIsTheEnd$0(java.io.ByteArrayOutputStream *, debug_symbols_smoke.ClassA *);
64:	void debug_symbols_smoke.Main::main(java.lang.String[] *);
49:	void debug_symbols_smoke.Main::thisIsTheEnd(java.util.List *);
 ==> expected: <true> but was: <false>
[INFO] 
Error:  Tests run: 18, Failures: 1, Errors: 0, Skipped: 5