Introduce #include <cosmo.h> to toolchain users

This change improves the way internal APIs are being hidden behind the
`COSMO` define. The cosmo.h header will take care of defining that, so
that a separate define statement isn't needed. This change also does a
lot more to define which APIs are standard, and which belong to Cosmo.

libc/log/log.h

@@ -43,8 +43,6 @@ const char *commandvenv(const char *, const char *);
 int LogKprintfToFile(const char *);
 const char *GetAddr2linePath(void);
 const char *GetGdbPath(void);
-void ShowCrashReports(void);
-int MakeProcessNice(void);
 bool32 IsDebuggerPresent(bool);
 bool IsRunningUnderMake(void);
 const char *GetSiCodeName(int, int);

libc/log/oncrash_amd64.c

@@ -226,7 +226,7 @@ relegated void ShowCrashReport(int err, int sig, struct siginfo *si,
       "  %s %s %s %s\n",
       !__nocolor ? "\e[30;101m" : "", !__nocolor ? "\e[0m" : "", sig,
       (ctx && (ctx->uc_mcontext.rsp >= GetStaticStackAddr(0) &&
-               ctx->uc_mcontext.rsp <= GetStaticStackAddr(0) + GUARDSIZE))
+               ctx->uc_mcontext.rsp <= GetStaticStackAddr(0) + APE_GUARDSIZE))
           ? "Stack Overflow"
           : GetSiCodeName(sig, si->si_code),
       host, getpid(), gettid(), program_invocation_name, names.sysname,

libc/log/oncrash_arm64.c

@@ -184,7 +184,7 @@ relegated void __oncrash_arm64(int sig, struct siginfo *si, void *arg) {
     gethostname(host, sizeof(host));
     reset = !__nocolor ? RESET : "";
     strong = !__nocolor ? STRONG : "";
-    if (ctx && (ctx->uc_mcontext.sp & (GetStackSize() - 1)) <= GUARDSIZE) {
+    if (ctx && (ctx->uc_mcontext.sp & (GetStackSize() - 1)) <= APE_GUARDSIZE) {
       kind = "Stack Overflow";
     } else {
       kind = GetSiCodeName(sig, si->si_code);