Fix Rust lexer buglet

Fix Rust lexer buglet

PR rust/24414 points out that the Rust lexer uses strtoul when lexing
an integer, and that this can give the wrong results in some
situations.

This patch changes it to use strtoulst, like most of the rest of gdb.
It also adds a self test.

Tested on x86-64 Fedora 29 using an i686 build.

gdb/ChangeLog
2019-04-09  Ivan Begert  <ivanbegert@gmail.com>
	    Tom Tromey  <tromey@adacore.com>

	PR rust/24414:
	* rust-exp.y (rust_parser::lex_number): Use strtoulst.
	(rust_lex_int_test): Change "value" to be LONGEST.
	(rust_lex_tests): Add test for long integer literal.