Bluetooth: Controller: Support Link Time Optimizations (LTO)

Support for using Link Time Optimization (LTO) when
building application with open source Bluetooth Low
Energy Controller. This reduces code space usage.

Current Memory Usage at the time of this PR (hci_ipc):

BT_CTLR_OPTIMIZE_FOR_SPEED:
Memory region         Used Size  Region Size  %age Used
           FLASH:      260112 B       256 KB     99.22%
             RAM:       60136 B        64 KB     91.76%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

BT_CTLR_OPTIMIZE_FOR_SIZE:
Memory region         Used Size  Region Size  %age Used
           FLASH:      232292 B       256 KB     88.61%
             RAM:       60128 B        64 KB     91.75%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

BT_CTLR_OPTIMIZE_FOR_APP_DEFAULT:
Memory region         Used Size  Region Size  %age Used
           FLASH:      232292 B       256 KB     88.61%
             RAM:       60128 B        64 KB     91.75%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

BT_CTLR_LTO:
Memory region         Used Size  Region Size  %age Used
           FLASH:      221484 B       256 KB     84.49%
             RAM:       60120 B        64 KB     91.74%
           SRAM1:           0 B        64 KB      0.00%
        IDT_LIST:           0 B        32 KB      0.00%

Signed-off-by: Vinayak Kariappa Chettimada <vich@nordicsemi.no>