PR go/89172
    internal/cpu, runtime, runtime/pprof: handle function descriptors
    
    When using PPC64 ELF ABI v1 a function address is not a PC, but is the
    address of a function descriptor.  The first field in the function
    descriptor is the actual PC (see
    http://refspecs.linuxfoundation.org/ELF/ppc64/PPC-elf64abi.html#FUNC-DES).
    The libbacktrace library knows about this, and libgo uses actual PC
    values consistently except for the helper function funcPC that appears
    in both runtime and runtime/pprof.
    
    This patch fixes funcPC by recording, in the internal/cpu package,
    whether function descriptors are being used.  We have to check for
    function descriptors using a C compiler check, because GCC can be
    configured using --with-abi to select the ELF ABI to use.
    
    Fixes https://gcc.gnu.org/PR89172
    
    Reviewed-on: https://go-review.googlesource.com/c/162978

From-SVN: r269266