ARM: introduce blind options to select & force THUMB mode

ARM: introduce blind options to select & force THUMB mode

Add three new blind options to set use of Thumb mode:
- COMPILE_IN_THUMB_MODE
  - if set, CFLAGS will contain -mthumb
  - if unset, the compiler's default is used
- HAS_THUMB
  - CPUS with Thumb instruction set can select this
  - use of BX depends on this
- FORCE_THUMB
  - CPUs that are Thumb-only must select this
  - this selects: HAS_THUMB, COMPILE_IN_THUMB_MODE and USE_BX

Also, remove leading space in Rules.mak.

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
Cc: Khem Raj <raj.khem@gmail.com>
Cc: Bernhard Reutner-Fischer <rep.dot.nop@gmail.com>
Cc: Carmelo AMOROSO <carmelo.amoroso@st.com>
Signed-off-by: Khem Raj <raj.khem@gmail.com>