avr32: fixup code style in unistd.h and syscall_table.S

This patch swaps the mix of tabs and space for alignment of comment
after code to use spaces only.

Also document why recvmmsg was defined twice in the syscall_table.S
table, but only once in unistd.h. In short, wired in the table by
generic arch patch, but forgotten in unistd.h (review slip).