[committed] pru: Document how -mmcu option uses MCU specs

[Dimitar Dimitrov <dimitar@dinux.eu>]

The plan to maintain PRU hardware-specific specs in newlib tree has been
abandoned in favour of a new distinct GIT project.  Update the
documentation accordingly.

gcc/ChangeLog:

	* doc/invoke.texi (-mmcu): Add information about MCU specs.

Signed-off-by: Dimitar Dimitrov <dimitar@dinux.eu>
---
 gcc/doc/invoke.texi | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)

diff --git a/gcc/doc/invoke.texi b/gcc/doc/invoke.texi
index 69020245b25..d75b28484bb 100644
--- a/gcc/doc/invoke.texi
+++ b/gcc/doc/invoke.texi
@@ -30106,8 +30106,14 @@ This is already the norm for most firmware projects.
 
 @opindex mmcu
 @item -mmcu=@var{mcu}
-Specify the PRU MCU variant to use.  Check Newlib for the exact list of
-supported MCUs.
+Specify the PRU hardware variant to use.  A correspondingly named
+spec file would be loaded, passing the memory region sizes to
+the linker and defining hardware-specific C macros.
+
+Newlib provides only the @code{sim} spec, intended for running
+regression tests using a simulator.  Specs for real hardware can be
+obtained by installing the
+@w{@uref{https://github.com/dinuxbg/gnuprumcu/,GnuPruMcu}} package.
 
 @opindex mno-relax
 @item -mno-relax
-- 
2.43.0