toolchain: Make it easier to build toolchains on modern MacOS

When building on MacOS:

 * greadlink instead of readlink
 * gsed instead of sed
 * $TMPDIR instead of a hardcoded /tmp
 * apply a macos-specific patch
 * additional option to GCC

Additionally, don't rely on the system 'isl' stuff; always have binutils
build its own.

The forum post also says that bumping mpfr 3.1.x to 4.1.x, and mpc
from 1.0.x to 1.2.x is needed, but I do not want to do that due to the
possibility of unknown codegen changes.

Change-Id: I18084d392fa927318cc2a1183da7eb2ffa0d5884

tools/toolchain-patches/apple_silicon.patch

@@ -0,0 +1,19 @@
+diff --git a/gcc/gcc/config/host-darwin.c b/gcc/gcc/config/host-darwin.c
+index 8f700eec85..b737abc006 100644
+--- a/gcc/config/host-darwin.c
++++ b/gcc/config/host-darwin.c
+@@ -22,6 +22,8 @@
+ #include "coretypes.h"
+ #include "diagnostic-core.h"
+ #include "config/host-darwin.h"
++#include "hosthooks.h"
++#include "hosthooks-def.h"
+ 
+ /* Yes, this is really supposed to work.  */
+ static char pch_address_space[1024*1024*1024] __attribute__((aligned (4096)));
+@@ -75,3 +77,5 @@ darwin_gt_pch_use_address (void *addr, size_t sz, int fd, size_t off)
+ 
+   return ret;
+ }
++
++const struct host_hooks host_hooks = HOST_HOOKS_INITIALIZER;