[mips] Correct tail call encoding for MIPSR6

r277708 enabled tails calls for MIPS but used the 'jr' instruction when the 
jump target was held in a register. For MIPSR6, 'jalr $zero, $reg' should
have been used. Additionally, add missing patterns for external and global
symbols for tail calls.

Reviewers: dsanders, vkalintiris

Differential Review: https://reviews.llvm.org/D23301

llvm-svn: 279064