[PATCH] fix symbol search loop at debuglink mode

[pavel-orekhov]

Some distribs like ubuntu'18 creatively name files with debug symbols.
(same name without .debug suffix). And give no build-id way.

$dpkg -L libc6-dbg | grep -v gconv
/usr/lib/debug/.build-id/68/f36706eb2e6eee4046c4fdca2a19540b2f6113.debug
/usr/lib/debug/lib/x86_64-linux-gnu/libc-2.27.so

$ dpkg -L libc6
/lib/x86_64-linux-gnu/libc-2.27.so
/lib/x86_64-linux-gnu/libc.so.6

$ objdump -s -j .gnu_debuglink -j .note.gnu.build-id
/lib/x86_64-linux-gnu/libc-2.27.so

Contents of section .gnu_debuglink:
0000 6c696263 2d322e32 372e736f 00000000 libc-2.27.so....
0010 5ac51387 Z...
Contents of section .note.gnu.build-id:
0270 04000000 14000000 03000000 474e5500 ............GNU.
0280 ce450eb0 1a5e5acc 7ce7b8c2 633b02cc .E...^Z.|...c;..
0290 1093339e ..3.

So elf_find_debugfile_by_debuglink("libc") loops back to
/lib/x86_64-linux-gnu/libc-2.27.so and can't lookup symbols for libc and
another standard libs. This patch breaks this loop and allows to lookup
for symbols at other places.

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[pavel-orekhov]

@googlebot I signed it!

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[pavel-orekhov]

FYA @ianlancetaylor