Yet another luajit.cmake unwind detection fix

[squeek502]

For reference, this is the logic we're trying to copy from LuaJIT's makefile:

https://github.com/LuaJIT/LuaJIT/blob/03080b795aa3496ed62d4a0697c9f4767e7ca7e5/src/Makefile#L339

---

Previously, CMake would only read up to the first NUL or control character (for me that meant it would only read "ELF" from tmpunwind.o). This was making CMake think there was no eh_frame/__unwind_info string in the file even if there actually was one.

Now, CMake skips binary data in tmpunwind.o and just reads all the ASCII strings from the file, which is fine for what we need to do with it.

This fixes:

In file included from luv/deps/luajit/src/ljamalg.c:23:
luv/deps/luajit/src/lj_err.c: In function ‘lj_err_unwind_dwarf’:
luv/deps/luajit/src/lj_err.c:469:2: error: #error "Broken build system -- only use the provided Makefiles!"
  469 | #error "Broken build system -- only use the provided Makefiles!"
      |  ^~~~~

for me on Linux x86_64 when trying to build Luv (unsure why this wasn't being hit in CI)

---

CMake MESSAGE output from when I was debugging:

Before (incorrect, TMPUNWIND_O should include more than just ELF and EH_FRAME_FOUND should not be -1):

-- TMPUNWIND_O: ELF
-- EH_FRAME_FOUND: -1
-- UNWIND_INFO_FOUND: -1

After (correct, eh_frame does exist in tmpunwind.o):

-- TMPUNWIND_O: ELF;>;@;@;UH;];GCC: (Ubuntu 10.3.0-1ubuntu1~20.04) 10.3.0;GNU;zR;x;E;C;K;%;tmpunwind.c;a;_GLOBAL_OFFSET_TABLE_;b;	; ;.symtab;.strtab;.shstrtab;.rela.text;.data;.bss;.comment;.note.GNU-stack;.note.gnu.property;.rela.eh_frame; ;@;@; ;;&;T;,;T;1;0;T;,;:;J; ;b;8;];@;8;; ;	;	;';P;l
-- EH_FRAME_FOUND: 215
-- UNWIND_INFO_FOUND: -1