MFC (1.7-1.8: install c++filt in the correct place).

The breakage is more tangled for RELENG_3. RELENG_3 still has
binutils/c++filt sources, so the elf case should have worked by
installing the binutils version in the correct place. However,
building of binutils/c++filt was broken in rev.1.4.2.1 of
binutils/Makefile by mismerging rev.1.8.

PR: 12920