mips: dl-machine-reject-phdr: Get rid of alloca.

Read directly into the mips_abiflags struct rather than reading the
entire segment and using alloca when the passed buffer is not big enough.

Checked with build-many-glibcs.py on mips-linux-gnu

Tested-by: Ying Huang <ying.huang@oss.cipunited.com>
Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>