[proposal] Clarify that AArch32 scalar polynomial types' underlying integer type is a recommendation

[vhscampos]

The definition of polynomial types in AArch32 is inconsistent between LLVM, gcc and ACLE.

At this point there's potentially high risk in changing the compilers' implementation as it may cause user code breakage.

The ACLE must be modified to state that the polynomial types are recommended to be unsigned, but ultimately it is up to the toolchains' discretion whether or not to follow it.