Provide reallocarray() stub in -legacy to allow building head on 9.x and 10.x

[dim]

Summary

After reallocarray() was added to head, building it on earlier releases
is likely to fail during the bootstrap-tools stage, with something
similar to:

===> usr.bin/m4 (obj,depend,all,install)
/usr/obj/home/dim/head/tmp/home/dim/head/usr.bin/m4 created for /home/dim/head/usr.bin/m4
[...]
/home/dim/head/usr.bin/m4/misc.c:357:12: warning: implicit declaration of function 'reallocarray' is invalid in C99 [-Wimplicit-function-declaration]
        void *p = reallocarray(old, s1, s2);
                  ^
/home/dim/head/usr.bin/m4/misc.c:357:8: warning: incompatible integer to pointer conversion initializing 'void *' with an expression of type 'int' [-Wint-conversion]
        void *p = reallocarray(old, s1, s2);
              ^   ~~~~~~~~~~~~~~~~~~~~~~~~~
2 warnings generated.
[...]
clang -O2 -pipe  -DEXTENDED -I/home/dim/head/usr.bin/m4 -I/home/dim/head/usr.bin/m4/../../lib/libohash -std=gnu99  -Qunused-arguments -I/usr/obj/home/dim/head/tmp/legacy/usr/include  -static -L/usr/obj/home/dim/head/tmp/legacy/usr/lib -o m4 eval.o expr.o look.o main.o misc.o gnum4.o trace.o parser.o tokenizer.o  -ly  -ll  -lm -L/usr/obj/home/dim/head/tmp/home/dim/head/lib/libohash -lohash -legacy
misc.o: In function `xreallocarray':
/home/dim/head/usr.bin/m4/misc.c:(.text+0xd65): undefined reference to `reallocarray'
clang: error: linker command failed with exit code 1 (use -v to see invocation)
*** Error code 1

This is because 9.x and 10.x do not have reallocarray() in libc yet.

To work around this:

• Add reallocarray.c to -legacy, and
• Add a wrapper stdlib.h which declares the function.

The latter is needed, because otherwise the compiler would assume an int
return value, which can cause segfaults on 64 bit architectures.

Test Plan

Build world from various older releases. I verified that this builds
from 9.3-RELEASE, at least to the cross-tools stage.