[test] Fix lld's ELF/linkerscript/thunk-gen-mips.s

[test] Fix lld's ELF/linkerscript/thunk-gen-mips.s

Summary:
Lld test ELF/linkerscript/thunk-gen-mips.s was accidentally disabled due
to the use of wrong FileCheck directives. As a result the test seems to
have bitrotted as it fails to pass if fixing the directive. To ease
updates to the test in case of change of the __start address the checks
have been changed to use numeric variables to express all the addresses
based on the __start address.

Reviewed By: atanasyan

Differential Revision: https://reviews.llvm.org/D79270