qt4: disable gdb_dwarf_index

* qmake is trying to call native gdb and we don't depend on gdb-native
  (or even provide gdb-native)
* fixes errors like this:
  /bin/sh: gdb: command not found
  /bin/sh: line 0: test: -gt: unary operator expected

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>
qt4: set QMAKE_COMPILER to gcc in mkspec

This avoids the following warning when running qmake:
Project WARNING: qmake spec does not announce the compiler family.
Guessed gcc.

Signed-off-by: Jonathan Liu <net147@gmail.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>