BR 2827397: fix invalid C in outcoff AddExports()

BR 2827397: fix invalid C in outcoff AddExports()

The construct:

         if (i == nsects)
             directive_sec =
                 sects[coff_make_section
                       (EXPORT_SECTION_NAME, EXPORT_SECTION_FLAGS)];

... where coff_make_section() can change the global variable "sects"
is undefined C, since there is no sequence point involved in the []
operator, and it is therefore fully permitted for the C compiler to
read the sects variable first.  Change this construct into two
statements to enforce defined behavior; this also ends up with the
code slightly simpler.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>