[llvm-readobj][test] - Move platform specific test cases and their inputs to separate folders.

This creates the next subfolders in the test directory:
"COFF", "ELF", "MachO", "wasm".

I've also removed platform specific prefixes, like "coff-*".
One unused binary was removed as well: `Inputs/relocs.obj.elf-mips`

Differential revision: https://reviews.llvm.org/D71203
diff --git a/llvm/test/tools/llvm-readobj/basic.test b/llvm/test/tools/llvm-readobj/basic.test
index 488a476..3f33ff6 100644
--- a/llvm/test/tools/llvm-readobj/basic.test
+++ b/llvm/test/tools/llvm-readobj/basic.test
@@ -14,8 +14,8 @@
 NO-OUTPUT-NOT: {{.}}
 
 # Test case where no switches are specified.
-RUN: llvm-readobj %p/Inputs/trivial.obj.elf-x86-64 | FileCheck --check-prefix=HEADER %s
-RUN: llvm-readelf %p/Inputs/trivial.obj.elf-x86-64 | FileCheck --check-prefix=NO-OUTPUT --allow-empty %s
+RUN: llvm-readobj %p/ELF/Inputs/trivial.obj.elf-x86-64 | FileCheck --check-prefix=HEADER %s
+RUN: llvm-readelf %p/ELF/Inputs/trivial.obj.elf-x86-64 | FileCheck --check-prefix=NO-OUTPUT --allow-empty %s
 HEADER: File: {{.*}}trivial.obj.elf-x86-64
 HEADER: Format: ELF64-x86-64
 HEADER: Arch: x86_64