RFR(XS) 8020791: [TESTBUG] runtime/jsig/Test8017498.sh failed to compile

[Calvin Cheung]

Please review this small fix for a testcase.

webrev:
     http://cr.openjdk.java.net/~ccheung/8020791/webrev/

Bug:
https://jbs.oracle.com/bugs/browse/JDK-8020791
http://bugs.sun.com/view_bug.do?bug_id=8020791

When the testcase was developed, it was using an incomplete jni_md.h
without the following:
#if defined(SOLARIS) || defined(LINUX) || defined(_ALLBSD_SOURCE)
The default typedef long jlong will be used during compilation of the
native code of the testcase on linux_x64.

With the above #if defined in the jni_md.h, if "LINUX" isn't defined,
the following typedef will be used and causing the compile error.
     typedef __int64 jlong;

The fix is to define LINUX (-DLINUX) in the gcc command line.
The change in TestJNI.c is just a minor cleanup.

Test:
    Ran jtreg on the testcase.

thanks,
Calvin