relax jalr $t9 [R_MIPS_JALR symbol] to bal symbol

[Alexandre Oliva]

The idea of the optimization is to take advantage of the R_MIPS_JALR
relocations emitted by the n32 and n64 assemblers to avoid pipeline
stalls due to dependencies between the instruction that loads the
symbol address into $t9 and the jalr instruction, that, because of the
way GCC generates code, will generally be adjacent.  If symbol is
reachable with a branch, and it's not overridable, we replace the jump
with a branch.  The transformation is only enabled when the --relax
flag is passed to the linker.  Tested on mips64-linux-gnu.  Ok to
install?

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-- 
Alexandre Oliva   Enjoy Guarana', see http://www.ic.unicamp.br/~oliva/
Red Hat GCC Developer                 aoliva@{redhat.com, gcc.gnu.org}
CS PhD student at IC-Unicamp        oliva@{lsd.ic.unicamp.br, gnu.org}
Free Software Evangelist                Professional serial bug killer