Bug 1918285 - Avoid lz4frame symbols being exported from gtest libxul…

…. r=firefox-build-system-reviewers,sergesanspaille

The lz4 symbols were never meant to be exposed, the API in Compression.h
being the official way to use it. This is we had LZ4LIB_VISIBILITY set to
nothing. Unfortunately, that wasn't enough, because there is another
similar define for lz4frame: LZ4FLIB_VISIBILITY.

So we had been exporting those lz4frame symbols from firefox-bin
forever, without noticing, but it didn't cause problems until the
symbols were moved to libxul. With them moved to libxul, we end up
with the situation where we might actually end up using the symbols
from the system liblz4, which is pulled indirectly through other
dependencies (through libsystemd, which comes through libdbus).

This is all fine-ish on a "normal" opt build, but with LTO, things
end up such that some calls go through our copy of lz4frame and others
through the system one, and the discrepancy causes a crash.

The symbols file for non-gtest libxul, that hides all symbols but a few,
was saving the non-gtest case, fortunately.

Differential Revision: https://phabricator.services.mozilla.com/D222574

mozglue/static/moz.build

@@ -21,6 +21,7 @@ SOURCES += [
 SOURCES["lz4/xxhash.c"].flags += ["-Wno-unused-function"]
 
 DEFINES["LZ4LIB_VISIBILITY"] = ""
+DEFINES["LZ4FLIB_VISIBILITY"] = ""
 
 # By putting it in js, it ends up in libxul too.
 FINAL_LIBRARY = "js"