amd64 string ops: use alignment more carefully, and comment it.

amd64 string ops: use alignment more carefully, and comment it.
By capping max padding to not be bigger than three next insns,
we avoid having ridiculously big NOPs like this one:

53:66 66 66 66 2e 0f 1f nopw   %cs:0x0(%rax,%rax,1)
5a:84 00 00 00 00 00

which was bigger than next three insns combined!

Size changes:

   text    data     bss     dec     hex filename
    102       0       0     102      66 x86_64/memcpy.o
    102       0       0     102      66 x86_64.old/memcpy.o

     90       0       0      90      5a x86_64/mempcpy.o
    102       0       0     102      66 x86_64.old/mempcpy.o

    210       0       0     210      d2 x86_64/memset.o
    242       0       0     242      f2 x86_64.old/memset.o

    213       0       0     213      d5 x86_64/stpcpy.o
    220       0       0     220      dc x86_64.old/stpcpy.o

    428       0       0     428     1ac x86_64/strcat.o
    444       0       0     444     1bc x86_64.old/strcat.o

    417       0       0     417     1a1 x86_64/strchr.o
    418       0       0     418     1a2 x86_64.old/strchr.o

     33       0       0      33      21 x86_64/strcmp.o
     33       0       0      33      21 x86_64.old/strcmp.o

    213       0       0     213      d5 x86_64/strcpy.o
    220       0       0     220      dc x86_64.old/strcpy.o

    135       0       0     135      87 x86_64/strcspn.o
    151       0       0     151      97 x86_64.old/strcspn.o

    225       0       0     225      e1 x86_64/strlen.o
    233       0       0     233      e9 x86_64.old/strlen.o

    140       0       0     140      8c x86_64/strpbrk.o
    156       0       0     156      9c x86_64.old/strpbrk.o

    135       0       0     135      87 x86_64/strspn.o
    151       0       0     151      97 x86_64.old/strspn.o

Also, a few files got their .text alignment relaxed from 16 to 8 bytes,
which reduces padding at link time.