Drop /root when installing symbols

This change fixes a problem that if a module is installed to the root
partition, its symbol file appears at under symbols/root, which is an
non-existing path at runtime. Since the symbol paths should match with
the runtime path, drop /root when installing symbols.

Bug: 123985838
Test: m libc.mountpoint
libc.so is found under symbols/bionic/lib[64]

Change-Id: I38bd226c3ec758d69902f4f288c83f68170c4068