Makefile: Fix building on AArch64 NixOS

The parsing of the output of archtest.c produced an unexpected
value on AArch64 NixOS. For example, the make variable ARCH was set to:

```
bit outside of fd_set selected
arm
```

This made the arch and OS checks fail.

This commit simplifies the parsing, making it more robust.

The C files archtest.c, endiantest.c and os.h used to set the
TARGET_OS, ARCH and ENDIAN variables, respectively, output
the result of the test as the final line, so just extracting
the final line and removing double quoting is enough.

This commit also fixes a bug with debug_shell lacking escaping
single quotes, which prevented using the single quote in the
debug_shell calls. It used to work by accident before this fix;
the line in the call happened to contain a balanced pair of double
quotes and lacked other characters that needed escaping, which
didn't break the debug_shell, but this was accidental and very
brittle.

Signed-off-by: Pyry Kontio <pyry.kontio@drasa.eu>
Change-Id: Iaa4477a71e758cf9ecad2c22f3b77bc6508a3510
---
M Makefile
1 file changed, 10 insertions(+), 6 deletions(-)