cmd/compile: use delta encoding for filenames in export data position info

cmd/compile: use delta encoding for filenames in export data position info

This reduces the export data size significantly (15%-25%) for some packages,
especially where the paths are very long or if there are many files involved.
Slight (2%) reduction on average, with virtually no increases in export data
size.

Selected export data sizes for packages with |delta %| > 3%:

                     package   before    after   delta     %

        cmd/asm/internal/arch   11647    11088    -559   -4%
   cmd/compile/internal/amd64     838      600    -238  -27%
     cmd/compile/internal/arm    7323     6793    -530   -6%
   cmd/compile/internal/arm64   19948    18971    -977   -4%
     cmd/compile/internal/big    9043     8548    -495   -4%
  cmd/compile/internal/mips64     645      482    -163  -24%
   cmd/compile/internal/ppc64     695      497    -198  -27%
   cmd/compile/internal/s390x     553      433    -120  -21%
     cmd/compile/internal/x86     744      555    -189  -24%
                     cmd/dist     145      121     -24  -16%
         cmd/internal/objfile   17359    16474    -885   -4%
   cmd/internal/pprof/symbolz    8346     7941    -405   -4%
      cmd/link/internal/amd64   11178    10604    -574   -4%
        cmd/link/internal/arm     204      171     -33  -15%
      cmd/link/internal/arm64     210      175     -35  -16%
     cmd/link/internal/mips64     213      177     -36  -16%
      cmd/link/internal/ppc64     211      176     -35  -16%
      cmd/link/internal/s390x     210      175     -35  -16%
        cmd/link/internal/x86     203      170     -33  -15%
                    cmd/trace     782      744     -38   -4%
                 compress/lzw     402      383     -19   -4%
                   crypto/aes     311      262     -49  -15%
                crypto/cipher    1138      959    -179  -15%
                   crypto/des     315      288     -27   -8%
              crypto/elliptic    6063     5746    -317   -4%
                   crypto/rc4     317      295     -22   -6%
                crypto/sha256     348      312     -36   -9%
                crypto/sha512     487      451     -36   -6%
                       go/doc    3871     3649    -222   -5%
    go/internal/gccgoimporter    2063     1949    -114   -5%
       go/internal/gcimporter    3253     3096    -157   -4%
                         math    4343     3572    -771  -17%
                   math/cmplx    1580     1274    -306  -18%
                    math/rand     982      926     -56   -5%
        net/internal/socktest    2159     2049    -110   -4%
                      os/exec    7928     7492    -436   -4%
                    os/signal     237      208     -29  -11%
                      os/user     717      682     -35   -4%
      runtime/internal/atomic     728      693     -35   -4%
         runtime/internal/sys    2287     2107    -180   -7%
                         sync    1306     1214     -92   -6%

                 all packages 1509255  1465507  -43748   -2%

Change-Id: I98a11521b552166b7f47f2039a29f106748bf5d4
Reviewed-on: https://go-review.googlesource.com/22580
Reviewed-by: Alan Donovan <adonovan@google.com>