Optimize dyld startup time by specifying a trivial export map

(only export main) on the mac.

This improves DYLD_PRINT_STATISTICS from:
total time: 6.0 milliseconds (100.0%)
total images loaded:  5 (4 from dyld shared cache, 3 needed no fixups)
total segments mapped: 0, into 0 pages with 0 pages pre-fetched
total images loading time: 0.0 milliseconds (1.4%)
total rebase fixups:  0
total rebase fixups time: 0.0 milliseconds (0.0%)
total binding fixups: 7,928
total binding symbol lookups: 4,087, average images searched per symbol: 1.9
total binding fixups time: 4.7 milliseconds (79.2%)
total bindings lazily fixed up: 170 of 4,372
total init time time: 1.1 milliseconds (19.2%)
total images with weak exports:  2

to:
total time: 1.4 milliseconds (100.0%)
total images loaded:  5 (4 from dyld shared cache, 4 needed no fixups)
total segments mapped: 0, into 0 pages with 0 pages pre-fetched
total images loading time: 0.0 milliseconds (5.7%)
total rebase fixups:  0
total rebase fixups time: 0.0 milliseconds (0.2%)
total binding fixups: 1,079
total binding symbol lookups: 75, average images searched per symbol: 1.0
total binding fixups time: 0.5 milliseconds (33.9%)
total bindings lazily fixed up: 14 of 216
total init time time: 0.8 milliseconds (60.0%)
total images with weak exports:  1

This reduces the time to -fsyntax-only cocoa.h with PTH from 0.192s to 0.184 (4.3%)

rdar://6505315

llvm-svn: 64882