release v2.1

Things of note:
- Don't cleanup /lib/libunwind.so (bug #667020)
- Drop GCC_PATH reconstruction (bug #174422)
- Use findmnt for mountpoint check when available (bug #693588)
- drop /etc/env.d/gcc/.NATIVE symlink
- drop /etc/env.d/gcc/config migration code
- drop empty /etc/env.d/05gcc-${CTARGET} files
- add einfo logging around library backup
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