Make PIC/PIE levels compatible for LTO.

The build system currently uses -fPIC for position independent
libraries, and -fpie for position independent executables. However,
these levels are incompatible and conflict when an executable includes
a static lib with the -fPIC flag - such as when building with
LTO. This CL changes the flag for executables to the compatible -fPIE
instead, which is equivalent for all architectures except PowerPC.

Bug: 30227045
Test: ENABLE_CFI=true m -j40 checkbuild # builds and boots
Change-Id: Ieb20bc2528300d88ff05e8787c5840ab6f03fb38