Hello,

Proposed 4 commits are code refactoring of boehm-gc (and libjava/boehm.cc, accordingly) to match API of recent BDWGC master ( https://github.com/ivmai/bdwgc ). This should simplify optional replacement of internal boehm-gcc with the external one.
All gcj tests pass.

Some of the benefits of using mainline boehm gc:
*  has no compiler warning
*  more targets supported (e.g. arm64)
*  no trouble with back-porting gc patches Similar thing was proposed for the Mono community -  https://github.com/mono/mono/pull/2247  (the patches were accepted recently)

References:
[1] The proposed patches in GCC unofficial git mirror fork -  https://github.com/ivmai/gcc/tree/match-bdwgc-api

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Best regards,
Ivan