libgcc: make heap-based trampolines conditional on libc presence

To build `libc` for a target one needs to build `gcc` without `libc`
support first. Commit r14-4823-g8abddb187b3348 "libgcc: support
heap-based trampolines" added unconditional `libc` dependency and broke
libc-less `gcc` builds.

An example failure on `x86_64-unknown-linux-gnu`:

    $ mkdir -p /tmp/empty
    $ ../gcc/configure \
        --disable-multilib \
        --without-headers \
        --with-newlib \
        --enable-languages=c \
        --disable-bootstrap \
        --disable-gcov \
        --disable-threads \
        --disable-shared \
        --disable-libssp \
        --disable-libquadmath \
        --disable-libgomp \
        --disable-libatomic \
        --with-build-sysroot=/tmp/empty
    $ make
    ...
    /tmp/gb/./gcc/xgcc -B/tmp/gb/./gcc/ -B/usr/local/x86_64-pc-linux-gnu/bin/ -B/usr/local/x86_64-pc-linux-gnu/lib/ -isystem /usr/local/x86_64-pc-linux-gnu/include -isystem /usr/local/x86_64-pc-linux-gnu/sys-include --sysroot=/tmp/empty   -g -O2 -O2  -g -O2 -DIN_GCC   -W -Wall -Wno-narrowing -Wwrite-strings -Wcast-qual -Wstrict-prototypes -Wmissing-prototypes -Wold-style-definition  -isystem ./include  -fpic -mlong-double-80 -DUSE_ELF_SYMVER -fcf-protection -mshstk -g -DIN_LIBGCC2 -fbuilding-libgcc -fno-stack-protector -Dinhibit_libc -fpic -mlong-double-80 -DUSE_ELF_SYMVER -fcf-protection -mshstk -I. -I. -I../.././gcc -I/home/slyfox/dev/git/gcc/libgcc -I/home/slyfox/dev/git/gcc/libgcc/. -I/home/slyfox/dev/git/gcc/libgcc/../gcc -I/home/slyfox/dev/git/gcc/libgcc/../include  -DHAVE_CC_TLS  -DUSE_TLS  -o heap-trampoline.o -MT heap-trampoline.o -MD -MP -MF heap-trampoline.dep  -c .../gcc/libgcc/config/i386/heap-trampoline.c -fvisibility=hidden -DHIDE_EXPORTS
    ../gcc/libgcc/config/i386/heap-trampoline.c:3:10: fatal error: unistd.h: No such file or directory
        3 | #include <unistd.h>
          |          ^~~~~~~~~~
    compilation terminated.
    make[2]: *** [.../gcc/libgcc/static-object.mk:17: heap-trampoline.o] Error 1
    make[2]: Leaving directory '/tmp/gb/x86_64-pc-linux-gnu/libgcc'
    make[1]: *** [Makefile:13307: all-target-libgcc] Error 2

The change inhibits any heap-based trampoline code.

libgcc/

	* config/aarch64/heap-trampoline.c: Disable when libc is not
	present.
	* config/i386/heap-trampoline.c: Ditto.