[gdb] Rewrite strings in producer_parsing_tests

I noticed some odd-looking strings in producer_parsing_tests, like this one:
...
    static const char extern_f_14_0[] = "\
Intel(R) Fortran Intel(R) 64 Compiler XE for applications running on \
Intel(R) 64, \
Version 14.0.1.074 Build 20130716";
...

Rewrite this in a more regular style:
...
    static const char extern_f_14_0[]
      = ("Intel(R) Fortran Intel(R) 64 Compiler XE for applications running on"
	 " Intel(R) 64,	Version 14.0.1.074 Build 20130716");
...

Tested on x86_64-linux.

Approved-By: Simon Marchi <simon.marchi@efficios.com>