Support for mapping symbol in RISCV.
There are three types in mapping symbols: $d $x $x<isa>.
For $d and $x is same as ARM, So i just copy related codes and testcases
For $x<isa>, the content of isa must same as Tag_RISCV_arch.
When emit Tag_RISCV_arch attribute, The following codes must have $x<isa> mapping symbol.
Adjacent same Tag_RISCV_arch attribute do not need two$x<isa>.
Here we have two testcases: mapping-symbols.s and mapping-isa.s.
mapping-symbols.s about the $d and $x in different segment.
mapping-isa.s about $x<isa> in different attribute Tag_RISCV_arch.
The mapping symbol in riscv references: