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docs: Reorder build options alphabetically

At present, build options in the user guide aren't listed in any
specific order. Ordering them alphabetically is a standard practice, and
is also easier on the reader.

Contents unchanged.

Change-Id: Ibc36f3a2a576edb86c1a402430d2ef5adcb2f144
Signed-off-by: Jeenu Viswambharan <jeenu.viswambharan@arm.com>