builtins: rework use of DEFINE_COMPILERRT_THUMB_FUNCTION

This is simply to help clarity of the code.  The functions are built as thumb
only if Thumb2 is available (__ARM_ARCH_ISA_THUMB == 2).  Sink the selection
into the location of the definition and make DEFINE_COMPILERRT_THUMB_FUNCTION
always define a thumb function while DEFINE_COMPILERRT_FUNCTION always selects
the default.

Since the .thumb_func directive is always available (at least on Linux, Windows,
and BSD), sinking the macro right into the macro works just as well.

No functional change intended.