Testsuite: Add gdb_simple_compile

Testsuite: Add gdb_simple_compile

Simplfy gdb.exp by adding a function that will attempt to
compile a piece of code, then clean up, leaving the created
object.

gdb/testsuite

        * lib/gdb.exp (gdb_simple_compile): Add proc.
        (is_elf_target): Use gdb_simple_compile.
        (skip_altivec_tests): Likewise.
        (skip_vsx_tests): Likewise.
        (skip_tsx_tests): Likewise.
        (skip_btrace_tests): Likewise.
        (skip_btrace_pt_tests): Likewise.
        (gdb_can_simple_compile): Likewise.
        (gdb_has_argv0): Likewise.
        (gdb_target_symbol_prefix): Likewise.
        (target_supports_scheduler_locking): Likewise.