Fix GDB build with GCC 4.8.2

Fix GDB build with GCC 4.8.2

PR build/27579 points out that the expression rewrite series
introduced a build failure with GCC 4.8.2.  The bug is that there's no
std::hash specialization for enum exp_opcode.  This patch fixes the
problem by using gdb::hash_enum.

2021-03-15  Tom Tromey  <tromey@adacore.com>

	PR build/27579:
	* rust-exp.y (maker_map): Use gdb::hash_enum.
	* stap-probe.c (stap_maker_map): Use gdb::hash_enum.