RFR: 7903920: Support building jtreg on FreeBSD

[snake66]

On FreeBSD we need to use GNU gmake instead of the BSD make program.

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Commit messages:
 - Defs.gmk: Use proper prefix for binaries on FreeBSD.
 - Defs.gmk: Use GNU grep on FreeBSD
 - Platform.gmk: Add support for FreeBSD as a target.
 - build.sh: Parameterize make binary.

Changes: https://git.openjdk.org/jtreg/pull/237/files
  Webrev: https://webrevs.openjdk.org/?repo=jtreg&pr=237&range=00
  Issue: https://bugs.openjdk.org/browse/CODETOOLS-7903920
  Stats: 32 lines in 3 files changed: 31 ins; 0 del; 1 mod
  Patch: https://git.openjdk.org/jtreg/pull/237.diff
  Fetch: git fetch https://git.openjdk.org/jtreg.git pull/237/head:pull/237

PR: https://git.openjdk.org/jtreg/pull/237