doc: mention STAGE1_CFLAGS

doc: mention STAGE1_CFLAGS

STAGE1_CFLAGS can be used to accelerate the just-built stage1 compiler
which especially improves its performance on some of the large generated
files during bootstrap. It defaults to nothing (i.e. -O0).

The downside is that if the native compiler is buggy, there's a greater
risk of a failed bootstrap. Those with a modern native compiler, ideally
a recent version of GCC, should be able to use -O1 or -O2 without issue
to get a faster build.

	PR rtl-optimization/111619
	* doc/install.texi (Building a native compiler): Discuss STAGE1_CFLAGS.