WIP: Graal native-image build of Java Schnorr Example

[msgilligan]

To build native image use:

./gradlew secp256k1-examples-java:nativeCompile

Build currently fails on macOS with:

Error: Support for the Foreign Function and Memory API is currently available only on the AMD64 architecture

Note: the implementation of FF&M (Panama) in native-image is incomplete. See: oracle/graal#8113

[msgilligan]

It also looks like some additional build configuration will be required:
https://github.com/oracle/graal/blob/master/docs/reference-manual/native-image/ForeignInterface.md

Update: The PR now includes this work (a somewhat rushed job of it, I must admit) but should build and run on AMD64 machines.

[schildbach]

As already mentioned elsewhere, I was able to build and run this successfully:

$ LD_LIBRARY_PATH=/nix/store/1c403w3lffz5p21lqqcvryh56ib0i28q-secp256k1-0.4.1/lib time -v secp256k1-examples-java/build/schnorrRunning secp256k1-jdk Schnorr example...
Is the signature valid? true
Secret Key: c6bf161b6530f588451b3790140722eb6de756164efc49baa1387f16b9089dac
Public Key (as ECPoint): 70176dcf4283032d089003785fce826ed7fa8e58c6a6a82ac27db4378d46ce50
Signature: 7ec2637314505842493670018a2e9b67ac0f63faf207640da4c206c2b2e89dcdaf0349608d40bec4c9f18ea2028eee70defd2e5bae1b7cada886ae88cfa7badd
	Command being timed: "secp256k1-examples-java/build/schnorr"
	User time (seconds): 0.00
	System time (seconds): 0.00
	Percent of CPU this job got: 100%
	Elapsed (wall clock) time (h:mm:ss or m:ss): 0:00.00
	Average shared text size (kbytes): 0
	Average unshared data size (kbytes): 0
	Average stack size (kbytes): 0
	Average total size (kbytes): 0
	Maximum resident set size (kbytes): 19520
	Average resident set size (kbytes): 0
	Major (requiring I/O) page faults: 0
	Minor (reclaiming a frame) page faults: 1225
	Voluntary context switches: 12
	Involuntary context switches: 1
	Swaps: 0
	File system inputs: 0
	File system outputs: 0
	Socket messages sent: 0
	Socket messages received: 0
	Signals delivered: 0
	Page size (bytes): 4096
	Exit status: 0