[MC] Set SHT_MIPS_DWARF section type for all .debug_* sections on MIPS

All MIPS .debug_* sections should be marked with ELF type SHT_MIPS_DWARF
accordingly the specification [1]. Also the same section type is assigned
to these sections by GNU tools.

[1] ftp.software.ibm.com/software/os390/czos/dwarf/mips_extensions.pdf

Differential Revision: https://reviews.llvm.org/D29789

llvm-svn: 297447