gprofng: avoid use of non-portable which [PR32166]

Distributions like Debian [0] and Gentoo are phasing out the use of
the non-portable `which` utility. Use POSIX's `command -v` instead.

[0] https://lwn.net/Articles/874049/

gprofng/ChangeLog
	PR gprofng/32166
	* testsuite/lib/Makefile.skel (JAVABIN): Replace use of which.