Work around MSVC linker out of memory failure

The native 32 bit MSVC linker tends to fail with LNK1102 (out of memory)
in debug mode. Work around this issue by disabling debug information for
x86, when not using the amd64_x86 cross-compiler.

Task-number: QTBUG-49545
Change-Id: I2aac7e400719f74ced450e264ab1464596e5f204
Reviewed-by: Michael Brüning <michael.bruning@theqtcompany.com>