testsuite: fix compiler vendor detection on dash as /bin/sh (#594)

testsuite: fix compiler vendor detection on dash as /bin/sh (#594)

In https://bugs.gentoo.org/753299 Paolo Pedroni reported
a single test failure out of all libffi. Here is the minimal
reproducer:

```
$ ./autogen
$ CONFIG_SHELL=/bin/dash ./configure --host=x86_64-pc-linux-gnu
$ make check RUNTESTFLAGS='complex.exp'
...
FAIL: libffi.complex/cls_align_complex_float.c (test for excess errors)
```

This happens because under 'dash' shell autoconf generates slightly
different style of string quotation in `config.log`:

- on bash: `ax_cv_c_compiler_vendor=gnu`
- on dash: `ax_cv_c_compiler_vendor='gnu'`

To avoid shell quotation parsing the change just embeds
`compiler_vendor` into `local.exp` at configure time.

Reported-by: Paolo Pedroni
Bug: https://bugs.gentoo.org/753299
Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>