Improved ARM cross-compile settings detection

Let the Chromium build system figure out default values if a flag can't
be deduced from the QMAKE_CFLAGS. For example, this allows Chromium to
default to ARM v7 if no ARM version can be deduced from compiler flags.

Change-Id: I47257a7fc27cdbca84c6999dba75affe809f753b
Reviewed-by: Andras Becsi <andras.becsi@digia.com>