Reduce memory footprint when linking

Linking QtWebKit on Linux x64 with debug symbols can use up to 11Gbyte
of memory. This can make it impossible to link on many systems.

This patch forces GCC 4.8+ to fully use DWARF-4 debug-types sections
to eleminate duplicate symbols and makes earlier GCC versions STABS
which also uses much less memory.

Task-number: QTBUG-36131

Change-Id: Ib1ba3e073b5984ead3a0badb935e7b89c01b962b
Reviewed-by: Jocelyn Turcotte <jocelyn.turcotte@digia.com>