Bug 1485210 - Add --sym-offsets=yes to valgrind command line. r=njn

When valgrind prints out backtraces, it prints raw addresses and symbol
names, but that doesn't help find the exact code that caused the errors,
because we don't know where the libraries are loaded.

With --sym-offsets=yes, it adds the offset from the symbol, which allows
to find the relevant code in the binary.

build/valgrind/mach_commands.py

@@ -114,6 +114,7 @@ class MachCommands(MachCommandBase):
 
             valgrind_args = [
                 valgrind,
+                '--sym-offsets=yes',
                 '--smc-check=all-non-file',
                 '--vex-iropt-register-updates=allregs-at-mem-access',
                 '--gen-suppressions=all',