aarch64: mpi: Fix building the mpi aarch64 assembly for windows

* mpi/aarch64/mpih-add1.S: Use ELF macro.
* mpi/aarch64/mpih-mul1.S: Use ELF macro.
* mpi/aarch64/mpih-mul2.S: Use ELF macro.
* mpi/aarch64/mpih-mul3.S: Use ELF macro.
* mpi/aarch64/mpih-sub1.S: Use ELF macro.
* mpi/asm-common-aarch64.h: New.

The mpi aarch64 assembly is enabled as soon as the compiler supports
inline assembly, without checking for .type and .size, as is done
for the rest of the assembly in cipher/*.S. (The .type and .size
directives are only supported on ELF.)

• Signed-off-by: Martin Storsjö <martin@martin.st>