SL-957: Explicitly pass VIEWER_SYMBOL_FILE from build.sh into CMake

instead of relying on both indra/newview/CMakeLists.txt and build.sh
generating the same file pathname.

Make build.sh set VIEWER_SYMBOL_FILE (instead of symbolfile) in pre_build, and
pass it to autobuild configure via -D switch. Then the uploads stanza can just
use VIEWER_SYMBOL_FILE instead of performing its platform-sensitive case
statement right there.

Introduce VIEWER_SYMBOL_FILE CMake cache variable, default empty string.

Make indra/newview/CMakeLists.txt generate_breakpad_symbols logic conditional
on VIEWER_SYMBOL_FILE being non-empty, as well as everything else. Eliminate
local set(VIEWER_SYMBOL_FILE) directives.