Add tls le relax related testsuites in ld.
The new test cases are mainly tested in three aspects:
1. tls le relax function correctness test.
2. tls le relax boundary check test.
3. tls le relax function compatibility test.
ld/testsuite/ChangeLog:
* ld/testsuite/ld-loongarch-elf/relax.exp: Modify test.
* ld/testsuite/ld-loongarch-elf/old-tls-le.s: New test.
* ld/testsuite/ld-loongarch-elf/relax-bound-check-tls-le.s: Likewise.
* ld/testsuite/ld-loongarch-elf/tls-relax-compatible-check-new.s: Likewise.
* ld/testsuite/ld-loongarch-elf/relax-tls-le.s: Likewise.
* ld/testsuite/ld-loongarch-elf/tls-relax-compatible-check-old.s: Likewise.
