Hi,

This patch adds non-relative jump table support for 64bit rs6000. It 
implements ASM_OUTPUT_ADDR_VEC_ELT and corresponding expansion of jump 
table instruction for 64bit rs6000. We want to put non-relative jump 
table in data.rel.ro section for rs6000. So I add a new target hook - 
TARGET_ASM_SELECT_JUMPTABLE_SECTION to choose which section jump table 
should be put in. It puts the jump table in data.rel.ro for 64bit 
rs6000. For other platforms, it calls 
targetm.asm_out.function_rodata_section, just as before. We want to have 
an option to use non-relative jump table even when PIC flag is set. So I 
add a new option - "mforce-nonrelative-jumptables" for rs6000.  It 
enables the feature by default. The option takes effect in target hook - 
TARGET_ASM_GENERATE_PIC_ADDR_DIFF_VEC.

static bool
rs6000_gen_pic_addr_diff_vec (void)
{
   return TARGET_32BIT || !rs6000_force_nonrelative_jumptables;
}

The attachments are the patch diff file and change log file.

Bootstrapped and tested on powerpc64le-linux-gnu with no regressions.  
Is this okay for trunk? Any recommendations? Thanks a lot.