mpi: enable assembler on all arm architectures

* mpi/config.links: remove check for arm >= v6
* mpi/armv6 => mpi/arm: rename directory to reflect that is is generic
  enough

MPI ARM assembly do not depend on CPU being armv6. Verified on PXA255:

Before:

Algorithm generate 100*sign 100*verify

RSA 1024 bit 3990ms 57980ms 1680ms
RSA 2048 bit 59620ms 389430ms 5690ms
RSA 3072 bit 535850ms 1223200ms 12000ms
RSA 4096 bit 449350ms 2707370ms 20050ms

After:

Algorithm generate 100*sign 100*verify

RSA 1024 bit 2190ms 13730ms 320ms
RSA 2048 bit 12750ms 67640ms 810ms
RSA 3072 bit 110520ms 166100ms 1350ms
RSA 4096 bit 100870ms 357560ms 2170ms

[ jk: ARMv6 => ARM in header comments ]

• Signed-off-by: Dmitry Eremin-Solenikov <dbaryshkov@gmail.com>