c++/1692: -fvtable-gc flag is only meaningful on ELF targets

[Gabriel Dos Reis]

"Joseph S. Myers" <jsm28@cam.ac.uk> writes:

| As a general matter, checking all PRs closed
| against both 3.0 and 3.1 might help find some regressions now rather than
| later.)

Furthermore, a corresponding testcase should be entererd in GCC's
testsuite, if possible.  That helps spotting regressions.

-- Gaby
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