XCOFF/AIX: Remove SEC_ALLOC flags for unmapped sections.

The .except, .loader and .typchk are not mapped to memory,
so do not set their SEC_ALLOC flag.

bfd/ChangeLog:

        * coffcode.h (styp_to_sec_flags) [RS6000COFF_C]: Add handling
        of STYP_EXCEPT, STYP_LOADER and STYP_TYPCHK sections.

ld/testsuite/ChangeLog:

        * ld-powerpc/aix-core-sec-1.hd, ld-powerpc/aix-core-sec-2.hd,
        ld-powerpc/aix-core-sec-3.hd: Adjust expected section flags
        for section .loader.