Allow manually setting the AArch32 instruction set

At the moment the AArch32 instruction set isn't specified in the command
line, which means that the compiler is free to choose the one it sees
fit. This decision may change between compiler versions, so it is better
to specify it manually.

The build option AARCH32_INSTRUCTION_SET has been introduced for this
reason. This option can be set to T32 or A32 to pass the correct flags
to the compiler.

The current behaviour is to default to T32 due to it's smaller size.

Change-Id: I02297eb1d9404b5868ff7c054fbff9b3cda7fdb6
Signed-off-by: Antonio Nino Diaz <antonio.ninodiaz@arm.com>