gdb: Allow prologue detection via symbols for Intel compilers.

gdb: Allow prologue detection via symbols for Intel compilers.

The next-gen Intel Fortran compiler isn't flang-based, but emits
prologue_end in the same manner.  As do the newer Intel C/C++ compilers.
This allows prologue detection based on dwarf for all newer Intel compilers.
The cut-off version was not chosen for any specific reason other than the
effort to test this.

gdb/Changelog:
2021-04-08  Felix Willgerodt  <felix.willgerodt@intel.com>

    	* i386-tdep.c (i386_skip_prologue): Use symbol table to find the
    	prologue end for Intel compilers.
    	* amd64-tdep.c (amd64_skip_prologue): Likewise.
    	* producer.c (producer_is_icc_ge_19): New function.
    	* producer.h (producer_is_icc_ge_19): New declaration.