Fix for test/jdk/sun/management/jmxremote/bootstrap/CustomLauncherTest.java on Alpine Linux

[Alexander Scherbatiy]

Hello,

The test CustomLauncherTest.java fails because it calls binary 
${platform}-${arch}/laucher file and there is no compiled binary file 
for musl libc library.

The proposed fix uses the built binary launcher file [1]. The binary 
file was compiled on Alpine Linux 3.11.3 (arch amd64) using jdk 13.0.2, 
cc 9.2.0, and musl libc (x86_64) 1.1.24 .

The CustomLauncherTest test is updated to check if musl libc is present 
and calling linux-musl-amd64/launcher file.

The code for musl library detection is moved from TestHelper to 
LibcUtils helper class.

http://cr.openjdk.java.net/~alexsch/portola/tests/jmxremote/webrev.00

The test with the proposed patch also passes on Alpine Linux 3.8.4 (arch 
amd64) with musl libc (x86_64) 1.1.19, and jdk 13.0.2.

[1] 
http://cr.openjdk.java.net/~alexsch/portola/tests/jmxremote/webrev.00/bootstrap/linux-musl-amd64/launcher

Thanks,

Alexander.