Small operands gcd improvements

[Torbjörn Granlund]

We might provide several gcc_11 function variants to accomodate the
internal uses you bring up.

gcd_1o1o - two odd limbs
gcd_1o1  - one odd and one odd/even limb
gcd_11   - two odd/even limbs

They would all be implemented in the same asm file.  In the absense of
asm implementation, C could provide just the most general gcd_11 with
tail callers for the other functions.

The speed difference will vary, I'd say 8 - 30 cycles for the initial
handling of potentially even operands.

-- 
Torbjörn
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