[mips] Add SHF_MIPS_GPREL flag to the MIPS .sbss and .sdata sections

MIPS ABI states that .sbss and .sdata sections must have SHF_MIPS_GPREL
flag. See Figure 4–7 on page 69 in the following document:
ftp://www.linux-mips.org/pub/linux/mips/doc/ABI/mipsabi.pdf.

Differential Revision: http://reviews.llvm.org/D15740

llvm-svn: 259641