RISC-V: Fix riscv gas/ld testsuites failures for big endian.

Add riscv_choose_[ilp32|lp64]_emul, and use them to choose the correct
linker script rather than set elf[32|64]lriscv directly.

gas/
    * testsuite/gas/riscv/li32.d: Accept bigriscv in addition
    to littleriscv.
    * testsuite/gas/riscv/li64.d: Likewise.
    * testsuite/gas/riscv/lla32.d: Likewise.
    * testsuite/gas/riscv/lla64.d: Likewise.
    * testsuite/gas/riscv/march-ok-g2.d: Likewise.
    * testsuite/gas/riscv/march-ok-g2_p1.d: Likewise.
    * testsuite/gas/riscv/march-ok-g2p0.d: Likewise.
    * testsuite/gas/riscv/march-ok-i2p0.d: Likewise.
    * testsuite/gas/riscv/march-ok-i2p0m2_a2f2.d: Likewise.
    * testsuite/gas/riscv/march-ok-nse-with-version.d: Likewise.
    * testsuite/gas/riscv/march-ok-two-nse.d: Likewise.

ld/
    * testsuite/ld-riscv-elf/ld-riscv-elf.exp: Added
    riscv_choose_[ilp32|lp64]_emul to choose the correct linker script.
    * testsuite/ld-riscv-elf/attr-merge-arch-01.d: Call
    riscv_choose_[ilp32|lp64]_emul instead of hardcoding elf[32|64]lriscv.
    * testsuite/ld-riscv-elf/attr-merge-arch-02.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-arch-03.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-arch-failed-01.d: Likewise.
    * testsuite/ld-riscv-elf/attr-merge-arch-failed-02.d: Likewise.
    * testsuite/ld-riscv-elf/c-lui-2.d: Likewise.
    * testsuite/ld-riscv-elf/c-lui.d: Likewise.
    * testsuite/ld-riscv-elf/call-relax.d: Likewise.
    * testsuite/ld-riscv-elf/pcrel-lo-addend-2.d: Likewise.
    * testsuite/ld-riscv-elf/pcrel-lo-addend.d: Likewise.
    * testsuite/ld-riscv-elf/weakref32.d: Accept bigriscv in addition
    to littleriscv.
    * testsuite/ld-riscv-elf/weakref64.d: Likewise.