refactor(cpufeat): feat detect helpers inlining

Force inlining feat detect helpers such that context save/restore
operations are flattened with sequences of ID regs read and conditional
instructions for system registers read/write. This is opposed to current
situation where with -Os optimization level, feat detect helpers get
called through non-inlined sequences of branch-link+ret.

Signed-off-by: Olivier Deprez <olivier.deprez@arm.com>
Change-Id: I2633442fb0e69e4a4ed13467e65846fb66d214f6