compilers: Introduce options for forcing inlining on SPL/TPL

There are cases where when we allow the compiler to decide about making
inline decisions rather than forcing them it can save us space.

For now, we keep the default values for inlining that we have had
historically.

Cc: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Tom Rini <trini@konsulko.com>