[mips] Removed the SHF_ALLOC flag from the .pdr section.

Summary:
This section is used for debug information and has no need to be
in memory at runtime. With this patch, LLVM now emits the same flags as 
the GNU assembler. This patch also fixes an error when compiling 
the Linux kernel, The error is that there are relocations within the 
.pdr section in a VDSO.

Reviewers: vkalintiris, dsanders

Subscribers: llvm-commits, dsanders

Differential Revision: http://reviews.llvm.org/D17199

llvm-svn: 260879