[mips][mc] Emit R_{MICRO}MIPS_JALR when expanding jal to jalr

When replacing jal with jalr, also emit '.reloc R_MIPS_JALR' (R_MICROMIPS_JALR
for micromips). The linker might then be able to turn jalr into a direct
call.
Add '-mips-jalr-reloc' to enable/disable this feature (default is true).

Differential revision: https://reviews.llvm.org/D55292

llvm-svn: 348760