Put YJIT into a single compilation unit

For upstreaming, we want functions we export either prefixed with "rb_"
or made static. Historically we haven't been following this rule, so we
were "leaking" a lot of symbols as `make leak-globals` would tell us.

This change unifies everything YJIT into a single compilation unit,
yjit.o, and makes everything unprefixed static to pass `make leak-globals`.
This manual "unified build" setup is similar to that of vm.o.

Having everything in one compilation unit allows static functions to
be visible across YJIT files and removes the need for declarations in
headers in some cases. Unnecessary declarations were removed.

Other changes of note:
  - switched to MJIT_SYMBOL_EXPORT_BEGIN which indicates stuff as being
    off limits for native extensions
  - the first include of each YJIT file is change to be "internal.h"
  - undefined MAP_STACK before explicitly redefining it since it
    collide's with a definition in system headers. Consider renaming?

test_asm.sh

@@ -1,7 +1,9 @@
+#!/bin/bash
+
 set -e
 set -x
 
-clang -std=gnu99 -Wall -Werror -Wshorten-64-to-32 yjit_asm.c yjit_asm_tests.c -o asm_test
+clang -std=gnu99 -Wall -Werror -Wno-error=unused-function -Wshorten-64-to-32 yjit_asm.c yjit_asm_tests.c -o asm_test
 
 ./asm_test