mips: Fix up mips*-sde-elf* build [PR112300]

mips: Fix up mips*-sde-elf* build [PR112300]

As reported in the PR, mipsisa64r2-sde-elf doesn't build because HEAP_TRAMPOLINES_INIT
macro isn't defined anywhere.
It is normally defined by
 # Figure out if we need to enable heap trampolines by default
 case ${target} in
 *-*-darwin2*)
   # Currently, we do this for macOS 11 and above.
   tm_defines="$tm_defines HEAP_TRAMPOLINES_INIT=1"
   ;;
 *)
   tm_defines="$tm_defines HEAP_TRAMPOLINES_INIT=0"
   ;;
 esac
in config.gcc, but mips*-sde-elf* is the only target which overwrites
tm_defines shell variable rather than just appending to it (or in one case
prepending), all other targets append something to it, including other
mips* triplets.
I believe (just from looking at config.gcc) that the difference is that
LIBC_GLIBC=1 LIBC_UCLIBC=2 LIBC_BIONIC=3 LIBC_MUSL=4 HEAP_TRAMPOLINES_INIT=0
isn't defined without the patch and is with the patch.

I think defining those first 4 shouldn't cause any harm and defining the
last one is required for it to actually build at all.

2023-11-27  Jakub Jelinek  <jakub@redhat.com>

	PR target/112300
	* config.gcc (mips*-sde-elf*): Append to tm_defines rather than
	overwriting them.