SOLVED. Confirmed working flawlessly in 1.6.4 RC2.
Debian Jessie, x64. Installing from *.deb packages.
Start-up checks go fine.
Try "Tools>Build Lazarus..." (no modifications, just clean Lazarus):
/usr/bin/ld: /usr/lib/fpc/3.0.2/units/x86_64-linux/rtl/cprt0.o: unrecognized relocation (0x2a) in section `.text'
/usr/bin/ld: final link failed: Bad value
make[2]: *** [lazarus] Error 1
make[1]: *** [idepkg] Error 2
make: *** [idepkg] Error 2
/usr/share/lazarus/1.6.4/ide/lazarus.pp(154,1) Error: (9013) Error while linking
/usr/share/lazarus/1.6.4/ide/lazarus.pp(154,1) Fatal: (10026) There were 1 errors compiling module, stopping
Fatal: (1018) Compilation aborted
Error: /usr/bin/ppcx64 returned an error exitcode
Makefile:4004: recipe for target 'lazarus' failed
make[2]: Leaving directory '/usr/share/lazarus/1.6.4/ide'
Makefile:4430: recipe for target 'idepkg' failed
make[1]: Leaving directory '/usr/share/lazarus/1.6.4/ide'
Makefile:3074: recipe for target 'idepkg' failed
make: Leaving directory '/usr/share/lazarus/1.6.4'
Identical error in "normal IDE" and "debug IDE" modes.
Also negative result for Lazarus 1.6.2+ FPC3.0.2.
Still Lazarus 1.6.2 + FPC3.0.0 works fine.
Lazarus-project 1.6.4 can't install with FPC3.0.0 due to dependency on FPC 3.0.2.
Looks like it depends on some external packages (like LibFreeType6). While it was very easy to solve at a work PC with Internet access: I've just (re)installed everything I had on my "compile.txt" list - it turned out to be a much more complex problem for my home PC without Internet access which already had all that list installed previously.
However, today I couldn't rebuild Lazarus again at work (I remember it was linking correctly Thursday or Wednesday).