MINOR: sym: also read the compressed symbol tables from the ELF files

MINOR: sym: also read the compressed symbol tables from the ELF files

Distributions ship libraries stripped, keeping local symbols in a
compressed .gnu_debugdata section. It is not loaded in memory, so it was
only reachable for objects mapping it in a segment of their own, i.e. the
ones we build, leaving system libraries unnamed.

Look for that section in the ELF files already mapped to read their
.symtab, and hand it to the same decompressor. A separate debug file still
wins as it is more complete, and the file mappings are released right away
since names point into the decompressed image. On Fedora this names
libcrypto, libssl, liblua, libm, libcrypt and libz, turning
"liblua-5:luaopen_package+0x41a0/0x205" into "liblua-5:luaV_execute+0x90f".